Well, you are, like I said, limited to an athlon 64 x2 CPU. With that line of processor as your only option, a higher end card like that HD4850, or 4870, will be bottlenecked by your CPU. Your best option is to settle for a mid-er range card.
Alternatively, you can buy a new MOBO+CPU+GPU for $200 (some mobo for $60, Athlon II, HD4670) that might make you happy. It's up to you on how much money you can spend.
Unfortunately, upgrading that CPU will be very difficult as the Socket 939 Athlon X2 & FXes are just eBay items now. I got an FX-60 for my A8N-SLI Deluxe right as the 939 was being completely put out to pasture. So, I'd suggest either getting a mid/low end GPU (4670 caliber, still a very nice card) or doing a full upgrade. With either choice, we'd be grad to help. The old 939 line is good, but it is really starting to show its age. I still use the FX-60 along with a 4850 as a secondary gaming comp, but the slow (and only 2GB ram) is really starting to take a hit, so over break/holidays I think I'll finally replace it for good.
It will be pretty hard to find a dual core socket 939 CPU. The price of the 4400 X2 939 is so insane it's cheaper to by a new AM2 board, RAM, and a cheap Athlon II CPU. Anyway I say get a Radeon 4850 as it's great for the price and you can use it when you get a new board and CPU. I guess you could also just go with a Radeon 4670 if you're not planning to upgrade the rest of your system anytime soon. Overclocking your CPU will certainly help.
Well, it was $350 back when it was released, so that is probably the best you can do on a non-production processor. It should be about the same in single threaded apps as you current processor, but much faster in multithreaded apps. Hopefully the 4670 should be a bit easier to get a hold of.