I'm currently experiencing low fps when playing Starcraft 2 with my new HD 5770. It runs perfect but every 5-6 seconds I get a quicck lag that lasts about a second. The reccomended settings according to starcraft 2 is extreme watever the most is. The fps range from 19-32 when playing. I don't understand. I have a dual core 2.8 amd processor. 5 Gigs of ddr2 ram. Windows 7 32bit. 1 TB Sata harddrive.
Also make absolutely sure you're not forcing AA through catalyst. Also SC2 is CPU heavy, and a 2.8ghz dual core probably is struggling. And finally, stutters like that can be caused by HDD access... if your HDD is more than 70% full then your access read/write speeds are going to be drastically lowered plus as pointed out with a 32 bit OS installed you can only use ~3.5gb of your RAM and if the RAM is filling up when gaming it's just going to compound the issue.