OK, few details, as others have said, a proc/mobo will be needed, i will suggest go the AMD way, so here is the idea,
Sempron 64 2500+ with some mobo available in market for about 4900 (I guess the mobo is ASUS K8N)
This is socket 754.
AMD Athlon 64 2800+ with ASUS K8N available in market for around 7-7.5,( i might be a little off on this one cost).
Decent graphics inegrated, u can barely play Far Cry etc.. i mean dont hope to play Far Cry ....
(Dont know what graphics it is)
This is socket 754.
AMD Athlon 64 2800+ with MSI RS480 available in market for around 9100.
ATI X300 GPU based graphics inegrated, play Far Cry, NFSUG2,....
This is socket 754.
AMD Athlon 64 3000+ with MSI RS480 available in market for around 10300.
ATI X300 GPU based graphics inegrated, play Far Cry, NFSUG2,....
This is socket 754.
(I will not recomment this, add a few moew bucks and go for the next combination)
AMD Athlon 64 3000+ with MSI RS480M2-IL available in market for around 11300.
ATI X300 GPU based graphics inegrated, play Far Cry, NFSUG2,....
This is socket 939.
(The one that i have, I'm satisfied)
If u have enough money,
U can get A64 3000+ for about 6700 in market
U can get A64 3200+ for about 9700 in market
Get asus/MSI nF4 ultra PCI-E motherboard, a good graphics card and get set go,...
And u have even more money, let ne know...