Hi,
I like to share to following scenario, hope it helps.
My PC: PhenomII .1Ghz, 2 GB RAM, ATI 4770, MS Vista
Ran SC2 at first, framerate was all over the place and at the beginning of each level, it slows down until 2FPS.

What I did was:
1. Increase RAM to 4GB
2. Defragment HDD
3. Most settings set to High

Voila!! Loading is much faster, and the FPS never slows down at the beginning.
Running now average 60ps everytime.

Dunno if it applies to any of you, good luck

Mostly set to Medium, but I think Lighting/Shader/Shadows causing FPS to drop a bit.. so I set them to Low..