The previous poster is correct, I use to run 2x 4850's. I had a cheap brand 750 watt PSU and things didnt go to well. Everything would work fine until a game actually pushed both cards to max at which time the entire computer would shut off. The 4850's were great cards in there time, but by todays standards they're power hungry. You would definitely need a PSU upgrade to run two of them, but by the time you bought a new PSU and another 4850 you would be almost to the price of a descent entry level gaming card (7750 or so).
Also, most games now are going beyond using 1 gig of VRam, the 4850's only having 1 gig (I only had 512 on my 4850's) will hold them back even though power wise 2 of them in crossfire isnt bad.