The two suggestions made so far are your best options. Twin 9600GTs would be slightly more powerful than a single 5770, but wouldnt offer Direct X 10 and would be a bigger strain on your PSU. (what do you have?) It may also matter what resolution you are running at.
Here is a comparison using tom's charts: (edit: sorry i had a bit of trouble getting it to internal link)
Hardware - Benchmark The Last Remnant
You can run a 9600Gt of another brand--but you should try to get the card's specs to match. (I'm not certain but I think some of the new low-power versions might be slightly different from an original 9600GT. )