I think its pointless to upgrade your from a C2D to another C2D. Overclock it instead and save your money on a better graphic card in the future.

Just play on high.

A locked multiplier, you can still adjust the FSB what. Unless your motherboard are those from Dell, Compaq, Intel etc.

No, you don't upgrade your processor just for playing SC2 alone. If you're gonna upgrade, make it well spent, go for a high-speed quad core but that's your money.

I don't understand how dual-core procs performs better, if you have equal speed, you might not be utilizing the extra cores on the quad-cores but definitely not slower.

A powerful graphic card might be what you need, or play in lower settings.