are you building your own or looking for a ready built low cost system?
The real gamers will tell you you need to spend enough to buy a low end pc on a graphics card but I wouldn't dream of it myself.
Older games will be fine on a geforce fx5200 or a readeon 9550 - I think these are the budget modern cards compatible with dx9
what's your budget? Mesh will do a nice whole system for about £650 delivered