A GTX480 as the best single GPU solution, you should be able to get a nice pre-overclocked one for that price, maybe even with a better cooler.
top performance will however come from buying two cards, if he has a board that supports SLi or Crossfire.
if so you then have a few tempting options, GTX460 SLi, which stock beat out a GTX480, but there are some insanely pre-overclocked ones availiable too (like EVGA FTW or TGT Beast) and they would be incredible performers.
then we have two new options from AMD although availiability may be scarce, either 2x6850 or 2x6870, both will run you under 500 USD and offer great performance too.