If you do go with the 4870, you might also want to bump up to a 750W PSU to give you a little headroom. I think the Corsair 650W would run crossfire 4870's, but it's cutting it a little close for my taste. Here's a couple options:
Ya, I think you'll be fine with two 4850's if you go that route.
Something you might want to also look into is the GTX 260 (216 core). Its about at par with the 4870 1gb, but a bit cheaper. I would do a little bit of homework and see what games benchmark better with what card. They are in general on the same level but ussualy the GTX 260 is a few dollars cheaper. Later on you can also install a second one and run these in sli and they do scale very nicely with the i7. Just something to think about though. Your setup is pretty good either way.
if you ever are thinking about running two cards you might want to consider bumping your psu up to the 750w verison. Gives you a little bit of breating room. (I'm pulling this out of my A$$) If i remember correctly two 4850's require 600w's which is borderline for even a good 610w psu. I just remember reading this on a forum post from someone that was benchmarking their system and testing how low they could go with their psu. So i really don't have any good sources for this info, other than it scares me now.