The motherboard was hugely upgradeable for something that was released in 2003, and the entire barebones system (ie no CPU/RAM/HD) was bought for a low 125 GBP at the time.
Although not in the user manual, it does support AMD Athlon XP-Mobile processors and the bios detects and displays this. Currently installed is an AMD 2800+ XP-M clocked at 2.13Ghz (Barton Socket A), with 16x multiplier. These (and similar supported CPUs) are getting quite cheap on Ebay now and are plenty strong for everyday use. Just for completeness it overclocks well up to FSB 145Mhz without changing any settings anywhere on the motherboard - should you feel the need.
It will take up to 2Gb Ram, which was plenty 4 years ago ..... and still plenty today!
There is also an AGP slot which works fine with the installed ATI Radeon 9800 Pro graphics card.... which by the way is also very cheap on Ebay nowadays. I don't envisage any problems with later chipsets up to the ATI X850 Pro / Nvidia 7800 GS if you require the additional oomph for gaming....
One issue is the stock power supply which can be insufficient depending on your specifications. If you search around on Ebay or elsewhere there are 250W and 300W replacements (original is 200W) by Shuttle. With no AGP card in the slot this is not an issue regardless of the CPU used - and to be honest... electricity is getting more expensive these days and it's good to have an economical and quiet PC...... you'd be very very hard pushed to find a desktop PC in the shops that uses a 200W PSU in most places nowadays !!!
The build quality has been solid all these years and shows no signs of giving up for at least quite a while.
It officially supports only IDE Hard Drives, but then again they come in 750Gb per drive at the time of writing.... so it isn't really an issue either. All in all, it will a lot longer yet.... considering that virtually no applications make use of dual cores or 64bit yet. If you really need SATA, you can always add a card in the PCI slot....
Other Downsides - Some of the onboard chipsets (audio/Lan) no longer have recent driver updates - but I've found the final ones to be relatively trouble free on Windows XP SP2. It's also a fiddly system to build and put together as it's small and compact.
A solid buy even today....
Update Nov 2008: A Radeon x1950 Pro 512Mb Graphics card was added a few months ago, and it works without problems on the 300W supply.
Update May 2011: Shuttle sold, an amazing 7 / 8 years of service. Hopefully new owner will continue to squeeze life out of it or use it for spare parts!