I don't think that you should invest into a PCI video card at all. It is the ultimate limitation of any computer that will be used for gaming. Many complain about agp video cards, but I believe that it is essential to have an agp at least if you expect to play games like WoW. I am running a 6600gt agp which is considered the best value gaming video card but I am able to run WoW blasted on max settings with hardly any lag.
With as little as $250 you can change your motherboard, cpu and graphics card. Just stay away from further bad investments into that pc by buying a pci video card.
GIGABYTE GA-870A-UD3 AM3 AMD 870 SATA 6Gb/s USB 3.0 Motherboard
NZXT Apollo Black ATX Mid tower Case
AMD Phenom II X6 1055t @ 3.8 GHZ
Rosewill RCX-Z1 CPU heatsink
Sapphire TOXIC Edition Radeon HD 5850 1GB @ 800/1175
2 x 2GB CORSAIR DOMINATOR 240-Pin DDR3 1600mhz
G.Skill Sniper SSD 60gb x2 RAID 0
BFG Tech GS-550 550W ATX12V V2.2 / EPS12V V2.8 SLI PSU