Upgrading Old PC, state of the art 2006, worth it?

[tfs104]

I have already put ~400 bucks into a new solid state hard drive, 8GB of memory, and Windows 7. Monitor showing slanted garbled image under high graphics load. Should I continue and upgrade the GPU next or quit while I'm ahead (out $400).

System:
Motherboard: ASUS M2N-SLI Deluxe
Processor: AMD Athlon 64 x2 4200+ 2.21 GHz
(4x2GB) 8GB Ram DDR2 - 800 (new)
NVIDIA GeForce 7900 GT/GTO
Crucial M500 240GB SATA (new)

I'm thinking of buying the Force 3D AMD ATI RAdeon HD 6450 2GB DDR3, graphics CArd for ~$70, will it work?

Thanks!

 

[tfs104]

Or should I start over with a new Mobo, processor, and Ram? I have a 600W coolermaster PSU