Starcraft 2 and a lot of newer games coming out are getting way more CPU intensive. If you on;y have a dual-core, you will notice huge lag spikes during big battles. I would recommend maing sure you have at least a triple-core to go with the 5770, then you should be fine. I remember the 57XX series didn;t have the best anti-aliasing though, so you might have to turn that off to get Ultra settings.
Actually a fast dual-core is better as SC2 only takes advantage of 2 CPU cores.