I upgraded my PC for sc2 last time.
i am using duo core but a bit higher then yours with a rm300 graphic card and 4 G RAM

I can play 4v4 with full blown out engagement without any problem. I am on low setting always and even playing skyrim my PC also is fine

Recommend you to get a high RAM graphic card and jab in another 2G RAM and you are fine to go.

I just tested ultra on a 4v4 map and it is smooth

OS: win 7, 32 bits
dual-core e6500 @ 2.93GHz (2CPUs)
Memory : 4096MB RAM
directx 11,
GC : Geforce GT220 2266MB

for your references