Force the game to run in 32bit DX9 (use Very High on DX9 tweak if you want eye candy, i use it).
There, fixed. :>
EDIT: whoa, maybe not. Magic San has a 3,6ghz 4 core cpu, you got a 2,6ghz 4 core cpu. That's 1ghz less. That's a lot.
EDIT2: e8200 @ 2,66ghz / 9800gtx / 4gb ddr2, very high, and my fps just randomly dropped to 4 on Marshes once twice. Ah well. I used to get 30-35fps on it in 0.2.0.