Your CPU should be fine. Sounds more like driver issues or something messy on your system. If you can't clean it up it a reformat would be the most drastic measure to improve performance.
Failing that the next best steps you could take to improve game performance would be to upgrade your video card and/or RAM to 2GB. Big open map slow downs sounds like somethings funky with your video drivers, perhaps you're running too high a res with too high AA/AF. Then there's the possibility of RAM being an issue with too much other stuff running in the backround and you're running out of system memory when you hit the big open maps forcing windows to page file to the HDD.
If you still want a CPU and are seriously considering putting down the money for as much as an FX, I'd say get the X2. Even if you had a single 7800GTX (which you don't, because you have a 6800GT), you'd still be fine with your 3500+. And X2 would be just as good for games and you'd then get the bennefits of dual core.