sticking within 500 with an 8800gt, vista and a tft you're left with 200 odd quid for a whole base unit. which is just daft. it's not really doable unless you go mega bargain basement 2nd hand parts. 
with that 650 spec you posted, you could save 30 odd quid by getting a 2180 cpu. but i can't see where else you can cut any corners.

with that 650 spec you posted, you could save 30 odd quid by getting a 2180 cpu. but i can't see where else you can cut any corners.

