How much are you looking to spend?
Newegg has a decent mobo/cpu combo here:
Newegg.com - Computer Parts, PC Components, Laptop Computers, Digital Cameras and more!
A good sound card is really cheap:
Newegg.com - Creative Sound Blaster Audigy SE 7.1 Channels 24-bit 96KHz PCI Interface Sound Card - Sound Cards
This is really all you need as far as sound cards...if you even need a sound card.
Like cybermike said, check out that video card thread for a good cheap one.
And good DDR2 ram is pretty cheap anywhere nowadays. Just grab a couple of 1gig sticks and you should be good in that department.
A decent mid-tower case with good airflow will only run you about $50. I recommend just using an old case that you have lying around and replacing the mobo and stuff in it. This way you can also just keep whatever cd-rom drives that are already in it.
Starcraft 2 won't take much to run on high settings. Blizzard tends to keep their games on the low end side so that everyone can play them. Hell, my old crappy computer that's 7 years old can run WoW on low-medium.
Building a good computer that will run even Crysis on high will only run about $400 max. I also recommend looking at....I think it's called hotdeals.com for good deals on some of these parts.