A good quality 450w PSU will be fine for the HD 4850. However, it also depends on what other components you have since they also consume power. For example, quad cores generally consume more power than dual core CPUs. For CF, I recommend a good quality 550w PSU.
Brands I recommned in order of my preference:
1. Seasonic (also builds some PSU models for #2 - #4)
3. PC Power & Cooling