You have either a 460W or 650w power supply so you can get a nice video card for it. Can you look on the power supply to verify? It apears to have 1 GB of memory so you could stand to upgrade that to 4GB.
Give us a budget for upgrade. If it were me, I would drop 4GB of memory (link below) and add the video card in the link below.
CORSAIR XMS2 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800
Newegg.com - CORSAIR XMS2 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model TWIN2X4096-6400C5
Here is a nice video card for the money but there are better.
EVGA 01G-P3-1366-TR GeForce GTX 460 SE (Fermi) 1GB 256-bit GDDR5 PCI
Newegg.com - EVGA 01G-P3-1366-TR GeForce GTX 460 SE (Fermi) 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
Here are the compatible CPU's for that Chipset.
CPU-Upgrade: Intel 955X Express chipset processor support