Here's a potted history of the Archimedes range by Darren Northcott and Matthias Siefert, now abridged by me:

As far as I'm aware the list of Archimedes machines went :
a500 (only 100 made), a305, a310, a410, a420, a440, a420/1, a440/1, a540,
a3000, a5000, A4 portable, a3010, a3020, a4000,
RiscPC, RPC600, RPC700, a7000, a7000+, RiscPCSA
Starting at the a305 with Arthur as the OS, with 512KB
RAM, 8Mhz ARM2 and a Single DD floppy drive.
The a310 was still Arthur, single DD floppy, but with 1MB
of RAM.

The 400 series first introduced RISCOS, the 2nd
digit in the number indicating the amount of RAM in MBs,
these machines had ST506 HDD interfaces built in, tho only
the 420 and 440 were sold with HDDs fitted.
The A440 (there was never really an A410 or A420 available) still was
running Arthur. In fact, the A305, A310 and A440 were introduced
(virtually) at the same time. These models were called "Archimedes",
i.e. "Archimedes 305", "Archimedes 310" and "Archimedes 440".
After the A3xx and A440 there was the R140. This was a slightly
modified A440. It was running a (disc based) Unix system called 
"RISC iX" (based on BSD 4.3).

After that, there was the A4xx/1 series (A410/1, A420/1 and A440/1).
The a4x0/1 machines fixed some design flaws on the 400
series motherboard.

These were followed by the A3000 (replacing the A3xx series). This was
the first model fitted with RISC OS (V2.00) right from the start (but
the old models could be upgraded to RISC OS). The A3000 was the first
model not called "Archimedes" anymore (AFAIK Acorn had problems
producing the A3xx series which lead in a whole production run being
dumped, which went through the [UK] press - and Acorn feared that this
gave a bad reputation for the "Archimedes" computers).

Then there was the A540. It contained an ARM3 which was clocked at 26
MHz and was fitted with a SCSI interface and a 100 Mbyte SCSI
harddisc. It was shipped with RISC OS 2.01 (which only differece
to 2.00 was, that it was able to handle more than 4 Mbyte of RAM as the
A540 could have up to 16 Mbyte of RAM).

The a3000 used an ARM2 at 8MHz, but was designed to be the 'entry level' 
games machine, RISCOS2, 1MB RAM, single DD floppy.

After that, there was the R260 and R225 which were based on the A540
and replaced the R140. They were both fitted with an ARM3 at 26 MHz and
built-in Ethernet interfaces. The R260 was also fitted with a SCSI
interface, a 100 Mbyte SCSI harddisc and 8 Mbyte of RAM, whereas the
R225 was discless (even without SCSI interface, the RISC iX OS was
booted off from Network connections) and only had 4 Mbyte of RAM.

The a5000 was designed as a partial replacement for the
a540, the first version had RISCOS3.00, which had a couple
of VERY nasty bugs in the filesystem, the second release had
ROS3.10, which was followed by RISC OS 3.11 (and RISC OS 3.19 which simply 
was the German variant of RISC OS 3.11). The 5000 had a 25Mhz ARM3 CPU, 12MHz memory,
2MB RAM as standard, 4MB max, IDE HDD interface, HD floppy drive, was sold in 
4 models with either 40, 80, 120, or 160 MB HDD.

The A4 was *supposed* to be the portable version of the a5000. 4MB RAM
was standard, 60 or 80MB HDD, 640x480 16greyscale LCD, 25MHz ARM3, RISC OS 3.11 
(with a few extensions in a seperate ROM).. The case was 'borrowed' from Olivetti 
(which held the majority of Acorn shares at that time). BTW: The A5000 was 
(according to people at Acorn) an A4 in a desktop box (i.e. it was a spin off 
from the design of the A4) - and not the other way round as one may expect.

The a3010, a3020 and a4000 were all released simultaneously,
all are based on the ARM250 CPU which was a single chip implementation of the ARM2 CPU, 
VIDC video processor, IOC and MEMC chips as found in the early 300 and 400 series machines. 
The CPU was clocked at 12MHz, the a3010 was marketed as the games machine, it had a joystick
port as standard, and also had a TV modulator, 1MB RAM, single HD floppy, no HDD, the 
a3020 had 2MB ram, came in 2 versions, one with no HDD, one with an 80MB HDD.

The a4000 looked like a cut down a5000, but actually had almost the same motherboard as 
the a3020, available in 2 versions, 2MB ram 80MB HDD, or 4 MB ram 160MB HDD. All used RISCOS3.10.

Then there was the A5000 (Alpha variant). It contained an ARM3 at
33 MHz and could be upgraded with a hardware floating point accelerator
(called FPA10).

The RISCPC was almost a completely new machine, it used a 33MHz ARM6 CPU, 160MHz VIDC20, 
enhanced IDE, new parallel and serial ports, and RISCOS3.5, typically about twice as fast 
as the a5000 in CPU terms, and over 6 times faster with video intensive tasks, this was the first
acorn machine to support 'truecolour' as standard, it also increased the expansion capabilities 
from 4 cards max in any of the older machines to 8 podules, 1 network card and a co-processor 
slot on the motherboard, the machine supported up to 256MB of RAM via 2 72pin SIMM slots, and 2MB
of VRAM in another slot, the case design is modular, each case 'slice' can take 1 5.25 inch 
drive and 1 3.5 inch drive, plus each slice adds 2 expansion card slots to the base machine,
the 'base' machine has a single slice, containing the HD floppy drive and two expansion slots, 
plus a free 5.25 inch bay. The HDD sits in a slot under the first slice, original machines
were supplied with 2 or 4MB of ram, 260 or 480MB hdds and 0 or 1MB of VRAM.

The RPC600 and 700 were improvements on the original RPC, the 600 used an ARM 610 processor 
clocked at 33Mhz and shipped with RISC OS 3.5. The 700 used a 40MHz ARM7 in place of the ARM6, 
because of internal CPU enhancements the 40MHZ ARM7 was almost twice as fast as the 33MHz ARM6. 
It was possible to retrofit 16 bit sound hardware into the 700.

The a7000 was a cheaper version of the RPC and a successor to the A4000. It utilised an
ARM7500 cell processor clocked at 32Mhz which contained the 4 main chips of the RPC700 in a single 
package, it had a simple non expandable case, and the design was such that you could have
'either' an internal CD, 'or' 2 internal expansion cards, but not both. The early a7000 used 
RISCOS3.5, later a7000s with 16bit audio used RISCOS3.6.

After that there was the StrongARM RiscPC (with just "RiscPC" written on its case). It was basically 
a RiscPC 700 with RISC OS 3.7 and a StrongARM at 200 (in fact 202.7) MHz. (There never was a StrongARM
with 170 MHz used in a RiscPC.)

The a7000+ was an enhanced version of the a7000, it used the ARM7500fe CPU which incorporated a 
maths chip, the first ARM CPU to do so, the clock speed was 40MHz, the memory bus was enhanced to 
allow faster rams to be used, and EDO was fully supported. The A7000+ always was shipped with 
RISC OS 3.71 (a variant of 3.7 with several bugfixes, which was never available for any other machine).

Then there was the StrongARM RiscPC (Alpha variant). It was identical to the previous StrongARM 
RiscPC but now contained a StrongARM with 233 (in fact 236.2) MHz. This machine was also sold 
under the name "J233" which only difference was, that there was additional software installed 
(including Java and a Webbrowser).

Then there was the RiscPC 700. This was basically a RiscPC 600 with an ARM710 (at 40 MHz), 16 bit 
sound hardware and a slightly better VIDC20 (which made it possible to display slightly higher 
resolutions). It was equipped with RISC OS 3.6.

Since the closure of Acorn in 1998, Castle technology has taken over manufacture and sales of 
the a7000+ and RPCSA, other companies have also started making RISCOS based computers, RiscStation, 
MicroDigital, and others. Development of RISCOS has been taken over by a new company called RISCOS Ltd, 
however, prior to Acorns closure Acorn sold the rights to RISCOS to PACE, as such most of the recent 
developments in RISCOS were done by PACE, and then released by RISCOS Ltd."