I use Intel Core 2 Duo E4500 Allendale. It has an operating frequency of 2.2 GHz, 2 MB cache, 800 MHz FSB and 64-bit ready. However since 64-bit performance is not only under doubt, but can actually hurt the performance, I am using it in 32 bit mode.
And since the others have also included their other system specifications, I will do the same:
Memory: Corsair XMS2, 2 x 1 GB, DDR 2 675.
Graphics card: EVGA 256-P2-N615-TX, uses nVidia 7600 GT chipset.
Hard drive: Seagate Barracuda 7200.10 ST3250410AS 250GB 7200 RPM 16MB Cache SATA 3.0Gb/s.
The operating system that runs all that is Arch Linux. Planeshift was easy to install with it, since it is in our community maintained repository.