So I'm going to buy an Asus 4850 off a friend for a decent price imo, and I was wondering if I could crossfire this card with a single-slot sapphire version? How many watts is would you recommend for a single card and CF?
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