Upgrade to 2x2GB RAM and to WD6400AAKS if you can.
Stick with the HD 4850, it's better bang for the buck than the HD 4870. To get a HD 4870 on that budget you'd have to make cuts elsewhere, like getting a crappy PSU or a case with bad cooling, or a slower CPU, etc. You're really better off with what you've got now.
Edit: OK, if you insist, you can have a HD 4870 on that budget if you get a GA-EP43-DS3L and a Sonata III with 500W PSU.
Thx for the replies guys, and yes aevm I was thinking abotu going with 4 gigs of RAM but i'm not sure if it will show much gain in my gaming or day to day use since i don't do much picture or video editing.
Uh yea I guess i'll stick with the 4850 then. I was thinking that instead of having my system able to do crossfire, i could save some money and get a single 4870 since i would prefer to have a single current gen. card in the future rather than have a pair of older cards. Note: I'm not sure that this mindset is correct and what the disadvantage of it is. If anyone knows please tell me.
No worries, there's no "correct" mindset. Just personal preference. In some games a pair of HD 4850 will beat a single HD 4870, while in others the single HD 4870 will win. Not that it really matters - performance and price are very close anyway.