The 805D would be a pointless upgrade--after the mobo, CPU, and cooling, and quite possibly new power supply required to get it running (unless you've got one that could handle SLI easily, since.. it peaked at over 400W, I think it was over 500W even, with a single graphics card, did it not?), you'd be over the 300$ you mentioned.
RAM and GPU. Go for a GPU upgrade. If you don't push any harder than 1280x1024, you won't see any visible difference in anything but MAYBE Oblivion. RAM's another fast way to increase your performance in some games, but your 6800U, it's gonna fall out faster than the 3200 is, which I hate saying since I've also got one. A 3200's a really good budget CPU.
If you're DETERMINED to get a processor instead of anything else, probably an X2 3800's your best bet, but.. really.. GPU and RAM, far better use of your money.