If you're looking to quadbox on a single pc, the above PC is not what you want. 1Gb ram is waaaay too little. You wouldn't be able to move a centimeter in a town. And a single core CPU would also be very stressful.

You need(want) 4Gb ram, not 1Gb. And preferably a quad core cpu, or -at least- a dual-core. Not a single core as the pc above.

Minimal to run 30 fps (15 fpsBK) running at decent enough settings not neccesarily highest but close too and good playability.
I'd go with dual core 2.66Ghz, 4gb DDR2, 512mb Geforce GTX (8800, or 9800) And a WD raptor/velociraptor harddisk or two as well as windows vista 64-bit.