As price seems to be the major concern I'd advise you to shop techbargains.com. Newegg and Tigerdirect and strike when you find the right card. You can often find deals at these sites. One note, I found a bargain GFX 470 at Ebay for $162 (shipping included).
I did some research and I found a pretty good car at 105$ with a 30$ rebate (thus making it 75) which is precisely what I need. The problem is that it uses a PCI-e x16 2.1 interface which may be incompatible with the 2.0 PCI-e x16 interface that the motherboard has (I have no idea if compatible). It is the
XFX HD-577A-ZNFC Radeon HD 5770 card ( http://www.newegg.com/Product/Product.aspx?Item=N82E168... )