I took an old P4 3GHz with 1GB RAM, Win XP Pro and put it in an Antec 300 illusion case with a good PSU and a 9800 GTX+ in the PCIE x16 v1 slot it has. It gets about 80% of the performance I got with my 3GHz athlon 64x2, 4GB, Vista 64 system. Good enough to play most games at 1680x1050, medium-high settings.
I dont know what games you want to play or what resolution the monitor is so I have no idea if a 7900GT is going to be powerful enough to make it worthwhile.
I can say the CPU will not be that limiting of a factor if the motherboard has a PCIE x16 slot for a graphics card.