You're limited by your PSU choices. You'll need a micro or mini ATX PSU. The biggest ones I saw on newegg were two 550W Athena Power units (both around 32 amps on 4 12V rails). Not a brand I'd normally recommend but you're really stuck by the form factor of that case. Both are over $100 ($115 and $125 IIRC). Everything else was lower power and would limit your GPU choice. The good part is both have power dangles for a GPU.
Either of these should allow you to run any card in the Nvidia 9000 series, even a 9800GTX+. At that point the question is how much you actually want to spend in total. I'd say a 9800GT at around $100 would be a minimum but if you can swing it get the 9800GTX+. Keep in mind you'll be pushing the power edge a lot closer with the GTX so if you want some overhead to spare then the GT might be the better choice at the loss of some game performance.
I'm not sure if you'll be limited to a low profile card or not. If you are then you're likely going to be stuck in the 9500GT range. At least then you have more options for lower priced PSU's.
Oh, and I'd check the measurements on those PSU's closely to make sure they'll fit.
