This review from July 2010 shows that Starcraft II favors Nvidia cards. At the time, AMD/ATI cards could not use AA settings, but that has probably been fixed by now.
The great thing is, that a lower end card should be able to run the game well, as long as you adjust the settings and lower AA. Unfortunately, your PSU is holding you back. Too bad you can't cough up ~$80 for a new one.http://www.guru3d.com/article/starcraft-ii-wings-of-liberty-gpu-graphics-performance/