|If keep that 2.6GHz Celeron D, there's no point in getting a video card faster than the GT 240 or HD 5670. |
To answer your question, a 450w PSU by Antec, Corsair, or Enermax will run up to a GeForce GTX 460 or Radeon HD 5850.
Super PIII | Unlocked ES Tualatin @ 1.8GHz (150x12, 1.65v, 512K L2)
2GB PC2700 | 500GB | Radeon x1950Pro | Apollo Pro 266T | Win 7 Pro