A "smooth 36fps"?
I dunno what is going on on your PC but you having some serious trouble somewhere.
Should be 1-3% CPU and 60fps solid. I have essentially the same configuration as you and I am getting much better results.
Maybe it's that 256KB of RAM.
Seriously,
I'd kill off all background tasks that it will let you and try running MA2 again. You are running it 1024x768 full-screen at 16-bit color on a single monitor, right?