what is your monitor's native resolution?
the issue is your video card, or lack of one. your CPU is more then capable for SC2
Nvidia has better performance in SC2 then ATI
at $85 (50 after the rebate)http://www.newegg.com/Product/Product.aspx?Item=N82E16814130535&cm_re=9800GT-_-14-130-535-_-Product
at around $120 you have the GTS250
at $200 you have the GTX 460 768mb
your PSU can run any of the cards above including the ATI 5770 ($160) which is an all around better card then the GTS250 but in SC2 their performance is almost equal.