Did some research (googling) on what it takes to run SC2 on Ultra, and according to 7th post here
all it takes is a 8800GT. So no need to get a 5830.
Here's an AMD build that brings the price down a bit:
CPU+Case = Phenom II X2 555 + NZXT M59
MOBO+RAM= Gigabyte GA-870A-UD3 AM3 + GSkill 4GB DDR3 133
GPU+PSU= Gigabyte 5770 + OCZ ModXStream 500W
Harddrive= Spinpoint F3 500gb
That comes out to ~$490 after rebates. You can cut more out of the budget by getting a smaller harddrive.
I found this review
of comparisons between the i3-540 and Phenom II x2. They're pretty similar in terms of performance (the i3 looks a little better), but considering sc2 doesn't have that high requirements anyway, you save $24 by going with the phenom. Your choice.
Did you need an OS and/or a monitor too? Wasn't sure since you didn't list those in your build.
Also in the future, it's helpful to fill out this form: http://www.tomshardware.com/forum/261222-31-build-advice,
as it gives people on this forum a better idea of what you want. But whatevers