I couldn't really say. I'm in a similar situation as you (with a smaller budget) and I'm looking at a Radeon HD 5670 with 1 GB of DDR5 ram for ~ $80 on Newegg.com and a 350W PSU that I already have (they cost about $20). My best advice would be to read over this http://www.tomshardware.com/reviews/fastest-graphics-card-radeon-geforce,3067.html
and browse over the graphics card section at Newegg until you find a nice match then pick a PSU based on the requirements of that card. Sorry I can't give you any specifics, I'm pretty new to all this, but I'm sure someone around here can be a little ore helpful than me.