500w Power Supply Unit will be enough, and will leave a little bit of room for expansion, not that you can get any better than an 8800 Ultra for your graphics card.
If you have the money, then Quiet PC do absolutely fantastic for Power Supply Unit's (Quiet PC Product Categories)
Otherwise, just try to buy a reliable make of PSU such as Silverstone or Thermaltake (Newegg.com - Thermaltake W0093RU ATX 12V 2.0 Version 500W Power Supply 115/230 V UL, CUL, TUV, FCC, and CB - Retail).



























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