you should've really posted the spec of that computer.
If the spec is any similar to this
Dell Inspiron 531 Specs - Desktops - CNET Reviews
(See? making everyone who want to help you go through more trouble...)
graphic card will help a bit,
but all the spec is really BARELY above SC2 minimum system requirements.
So if you upgrade the GPU, you will see a slight increase in performance, but before it can do all it can,
you will be bottlenecked by your CPU or RAM.
bottom line, that computer just isn't very good for playing SC2.