It depends how much you want to pimp it out. I don't know what the requirements are for Star Craft 2.
First I would toss more RAM in there 1gb isn't very much these days, I would say 4-6 gb of Ram is a good amount
Second get a video card. I think the 6150 is onboard graphics. You can get a decent video card for 100-150 bucks. The top of the line ones are closer to 600, but again it depends on how demanding the games are that you want to play.
Third you can think about upgrading the processor, that one isn't terrible but there are better ones out there. If the computer is 3 years old you might have a socket 939 motherboard, in which case you processor upgrade possibilities are quite limited
Like I said it depends on what you are looking for in increased performance. If you just want to get a little more juice out of it, more ram and a video card is where you should start.
Here is what you would be looking at
http://www.memoryexpress.com/Product...9/Default.aspx
and
http://www.memoryexpress.com/Product...3%28ME%29.aspx
These 2 upgrades For $210 would be a significant increase over what you have right now