Well if you want the absolute best performance for the cheapest buck, we need to know what processor you're running etc. The reason for this is that we know it is an AMD proc, but if you have that at stock speeds you could seriously bottleneck high end cards. Lemme know what other specs you gots and what rates everything is running at.
Well dude, what type of board do you have? Also, maybe you want to overclock the proc a bit too. Futhermore, if you do that you won't need new ram. Trust me, you are better off getting a new rig. I'm in your same position. What I would do personally is make sure you have a nice PSU. Then I would get a ATI 4850. For the price it rules. Use that to wait out a computer upgrade.
You need to look at how to harness the power of your 4800+ by going to forums on how to overclock with your Motherboard.. I know you can hit at least 280 on the HTT @ x4.. you might be able to get that processor to 3.2-3.4 using that processor and RAM if your lucky..
you don't need to upgrade anything, just overclock and keep your RAMs timings/latencys correct.
HTT x 4 (800)
add some CPU voltage (+100/+200)
I've got the Asus A8R32-MVP Deluxe and i am getting a new PSU so i can crossfire my 3 year old system with another 3870 in another 6 months.. (probably not worth it, but it will look sick..)
check out the toms charts for 3d performance of graphics cards
I would still say the 4850. A few reasons. First, you don't have a hefty PSU. Second, cpu will bottleneck anything much bigger. Lastly, you don't even play newest games.