Video Card Help

Okay so i am looking to build new rig still. I have made a couple posts getting some info on parts for the comp. I was wondering what the consensus is on video cards right now. I am looking at spending about 350-400 on the video card set up. My question i guess is this: is it better to go with 1 higher end card (HD 4870) or 2 slightly lower end cards (2x HD 4850). What kind of performance differences will I see with the 1 vs 2 card set up?

thanks in advance for the help

From the research I did it seemed like more people recommended one HD4870. You can then go back and add another HD4870 later if you want.

Not sure if i should have made this clear but i would be running with 2 monitors. Not sure if there is any performance issues with send info from one card to 2 monitors or better if you send 2 cards to 2 monitors.