The Shuttle SN25P is an excellent gaming PC when equipped with a filthy fast dual-core CPU, a mean graphics card and cooled, dual-channel RAM.
World of Warcraft, Half-life 2, Grand Theft Auto III, Homeworld 2, Ghost Recon and many others all run with maximum detail (including anti-aliasing in some cases) at 1280x1024 at 60+ fps. You will find lots of reviews online that detail performance and such.
However, nowhere does it say that the BIOS does not have a quick boot menu. If you want to boot from another disk or the network, you have to go in and change the setting each time. Slightly annoying if you want the default boot to be a gaming OS and you sometimes want to be able to do some actual work on the machine in another OS.
2GB RAM isn't going to last you if you want to be playing the latest wicked games in a year or two from now (March 2006).
Also, long, dual-slot graphics card do fit but you need to be a little creative and apply just the right amount of force. You might need to move the duct fan out of the way while installing, say, an ASUS/ATI EAX1800XT. Shuttle do not seem to publish the maximum size of video cards for each model of machine.