I agree about th 9600gt, it is a downgrade essentially. I work on computers for a living on both PC's, as well as have my Apple cert. There is about a 15-20% performance difference in favor of the 8800gt vs the 9600gt if memory serves. Don't get me wrong, 9600gt is not horrible, I have one myself, but going from the 8800gt to the 9600gt is the equivalent of downgrading. If I remember right, the 8800gt has 112 stream processors vs the 64 of the 9600gt which makes up a lot of the difference.
For the PCI 1.0 vs 2.0, 2.0 might help, but I am not sure many cards actually can saturate the full bandwidth offered by 1.0-1.1. At least I believe that was the case a few months ago, but who knows with the new 5870 series. That being the case, my next upgrade for graphics will likely be a 5750 or 5770. However, the op may be able to find a cheap 4850 floating around somewhere, but at this point, I think I'd go ahead and jump to the DX 11 cards, but that is a personal preference.