i already have a 3870. Would it be a better idea to get get another 3870 and CF or wait till till the 1gb 4870? I wanna play S.T.A.L.K.E.R Clear sky on full settings @ 1680 x 1050. Another 3870 would cost around $130 while the 4870 is around $300. Which would be better? Oh yea, is their a way to update your windows xp to 64 bit with out doing a fresh copy?

I can't really say anything about the video card deal, but for the "Oh yea, is their a way to update your windows xp to 64 bit with out doing a fresh copy? "

The answer is a sad sad no
One of the reasons I went with Vista is that since I had to do a fresh install and I wanted over 3.5GB's of RAM I figured might as well hit DX10 while I am at it. I didn't really want to upgrade to XP 64 then a year down the road say "Oh now drivers are better for Vista then XP hmmm time to reinstall again"

My guess is that a 3870 CF might come close to maxing clear sky if the devs do a good job on CF support. Most likely it won't. I am sure a 512meg 4870 will do the job. At 1680 x 1050 you shouldn't run out of Vram.
good luck man!

HD4870 has more advantages than my 3870