If you want to go nvidia, I'd say the GTS 250. ATI, go with a 5770 or a 4870.
If it were my money, I'd get the 5770, as it is very close to the performance of a 4870, uses less idle power, runs cooler, and has DX11.
Thanks for the advice, I think I might get the 5770. I considered it before, but it's hard to dish out 165$ min at 9$/hour at bk, especially when the only booked me for 8 hours this week!