The Shuttle XPC that we used with the iz3D Gaming Monitor deserves a mention as well. The XPC P3100 unit is quite small and generates barely any noise, thanks to its Silent X technology.
Inside, there's a 3.4GHz Pentium 4 550 LGA775 processor, a 150GB SATA Western Digital Raptor, 1GB of DDR2 Kingston RAM, 16x DVD+RW, and a PNY nVidia 7600 GT video card. It powered the games we tested on it just fine—but they were a year or two old, so we'd be surprised if this didn't run the games well. The XPC did a good job generating the dual images for the iz3D monitor with no slowdown at all, which is impressive.
We ran 3DMark 06 on it, and it got a score of 2870 SD Marks, with an SM2.0 Score of 1218, HDR/SM3.0 Score of 1132, and a CPU Score of 1017. Comparing to scores on the net, it's decent but not cutting edge.
There are a slew of USB and firewire ports on the front and back of the unit, but honestly you won't have problems reaching around the small machine to plug in stuff into the rear. There's also an included card reader, and various connectivity options. A good amount of power in a little package.