To the OP. I'd be tempted to stick with the 4850 for now seeing as you're playing at a low resolution then seeing how it does. Graphics card prices are heading south at the moment anyway with potentially new ones coming out soon - you've got nothing to gain by changing now.
As far as the CPU goes, try picking up a second hand Q6600 (£50-60) and seeing how it goes. If it doesn't work out you're not going to lose much by selling it on again.
I imagine the final game will work differently to the beta, so just see how it goes. Patches in the first few months will change things too.