I see your point but that Phenom ii x4 820 is going to severly bottleneck a GTX570 at any resoultion.I think maybe a cheap alterative until you have enough money to get a whole new build would be best.A GPU like the GTX570 is way to much money to spend on an already aging system.Espically if your not going to upgrade for another year.By that time the GTX570 will be old news and starting to show it's age.
Since your mobo is PCI-Express slot is only 1.0 I would stay away from 2.1 cards as they are known to have issues with 1.0 slots even tho they are backwards compatible.
Here's what I think would be best for you...
MSI Cyclone 6850(PCI-Express 2.0) $175=$160 after MIR + Shippinghttp://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=7192175&CatId=3669
That card is 2.0 so you shouldn't have any problems with combatility.It should also last you until your next ugprade without bottlenecking to much.Overclocking your CPU is best to limit the bottleneck to a minimum.