So, my poor desktop running WinXP built during college for $300 is too crappy to really run Starcraft II.
Ended up defragging my MacBook Pro (4 GB of ram, original GeForce 8600M GT, Intel Core 2 Duo 2.2GHz), installing Bootcamp, installing WinXP SP2, discovering that I couldn't install SP3 or anything else because unpacking SP3 ate up way too much room. Scrapped the Bootcamp partition, recreated it at 15 GB, and re-did everything.
(By the way, company that made iDefrag: Thank you. Without you, I wouldn't have been able to get Bootcamp to partition my drive (it kept complaining that there was a file it couldn't move. Booting into my backup firewire drive and running iDefrag in Compress mode did the trick). You are officially more than well worth whatever I paid you guys.)
Note: I downloaded Starcraft II previously on my desktop system. I have about 4 GB free on my MacBook Pro's Bootcamp partition after installing it (and Firefox and Flash). So if, for some odd reason, you're reading this and need to do an installation yourself, you'll probably need at least 13 GB free, although that may be cutting it kinda close.
Did a few battles with the really easy CPU, then got my ass handed to me against a real human player. And then turned around and slaughtered some poor zerg player. The protoss ability to Blink (Stalkers) is scary, especially when it comes to newbie maps where you have rocks that you must destroy. Blink essentially lets you jump over any obstacle in your way and just skip around a nasty group of close combat melee units like Zerglings or Zealots.