Your links are not working quite right. However, your build is actually very similar to what I am working on for myself right now (UD3H + 965) Depending on your budget there is an HD Radeon 5850 on sale for $270 right now if that would work for you. If you did not have your processor I would have recommended this even more strongly since you can get a combo giving you $20 off. Here is the 5850 though, incase it can fit your budget:
The 5770 should be fine for SC2, even at 1920x1080. I'll have to dig up the link, but I believe that with an i7-930 and a 5770, SC2 at 1920x1200 Ultra details was doing about 45 fps. Even if you want to knock off a very generous 1/3rd for the difference in CPUs, you'd still be at a playable 30 fps. And if you're gaming at a lower resolution, that will increase your fps compared to 1920x1080 anyway.
750W is definitely overkill for what you have right now. It's probably still overkill for 2x 5770. You could potentially drop down to 650W, but if you want headroom just in case, the 750W is fine.