if your willing to build, a basic but decent gaming machine can be had....
it wont be the total dogs b's right now, but it can be upgraded in future for games like Crysis to run smooth
go to overclockers.co.uk and spec it up...
Antec Sonata II case with 450watt PSU £70
C2D bundle (6300, Gigabyte DS3 Mobo, 1Gb Geil RAM) £266
any HDD you prefer £40 approx for 160-250Gb
likewise and dual layer DVD-+RW £20
X1950 Pro approx £100 (I think they have one that price on offer this week)
£500 bingo, it'll do the business, and when DX10 cards come down in price in future you can upgrade that, the Mobo will also take a Quad core upgrade in future too, although you can usually overclock the CPU you get quite happily to a very nice speed (i got unlucky and i dont think mine will overclock much but its speedy anyhow compared to my P4 3ghz...lol)
edit: btw, the specs on that ebay one, Dual 3600? I may have missed it but i thought AMD X2's started at 3800?