GPU
Radeon HD 4870
Radeon HD 5850
Radeon HD 5870
Max. Board Power (TDP)
160W
151W
188W
respectively
That being the Power consumption, depending on your other components and future upgrades, you need to choose a power supply. If you’re getting the 5850, then I assume that you wouldn’t be upgrading the GPU at least for a year. So, that being the case a decent 450W or higher power supply should work fine. You should start with the Corsair VX450 and look higher. Anything rated higher than this is fine as long as it delivers what it is rated at. The local PSUs rated at 500W may actually give only 250 or 300W which then becomes a huge problem. So, choose a PSU that is 80PLUS certified.