We need to know for what kind of a slot, pcie, agp ?
I forgot to say that the processor is Pentium 4 HT, i dont know if that help
You have to find out if your mother board accepts pci-e DDR3 -- assuming it is OK, then you can get for around $55 an Radeon HD 6670. You should be able to play quite a lot of games at a moderate settings.
And I dont have a bottleneck? And all together work good?
His PC will not support HD 6670, which is a PCIE 2.1 card. He has PCIE 1.0 slot. Anyway, I'd suggest GT 430 for $50: