My recommendation is to just go with 1 Ati 4850, see how you like it, and if the games you play most support crossfire well, then you can always upgrade later. The SLI and CF benchmarks show that you don't automatically get performance gains just by adding another card in SLI. The software has to support it. For example, I love Flight Simulator X, but adding another 4850 to my rig will not help my framerate, at all. Which sucks, because I'd love to do just that.
For reference, here's my rig:
Intel E8500 running at 3.8ghz
4 Gigs DDR2-800
windows vista home prem. 64bit
WD - 7200 rpm - 150Gb hard drive
Panasonic Blu-ray drive
Samsung 22" 1600 x 1050 monitor
Another note, I have an Epson HD projector (for movies) and it works flawlessly with either the HDMI connector that comes with the 4850 or standard, VGA monitor cable. I use the monitor cable because it cost me a whopping $20 for a 15' cable, compared to $50+ for an hdmi cable. newegg rocks for all of the above.