You have a file called variables.txt in your starcraft 2 folder (inside My Documents). You need to add the following lines to the file, and save it.

frameratecap=60
frameratecapGlue=30

These lines (if you can't tell) cap the framerate in the game (the first line) and in the menus (second line), and will keep your card from overworking itself to produce graphical perfection that you can't perceive. It fixes the problem for most people. That said, your system isn't particularly high end, and I wouldn't think it'd be churning out frame rates much higher than the above anyway. Hmm.

Alternatively, set the graphics down, like you were planning to, and pay special attention to the shader settings. SC2 goes heavy on the shaders, and that's pretty intensive work for your card.

If you want to monitor how things are going, download speedfan, or another similar program. If you can hook up an external monitor to watch it on, great. If not, just alt-tab out of the game every few minutes during your first play session to watch the temps. As to the 81c temperature, which sensor was that? That's not TOO bad if it's a video card. That's high for most cards, but not dangerous for very many. A lot of ATI cards are actually designed to run fine in the low 90s. If it was your cpu, that's slightly more of a problem, and more likely to cause a reboot.

EDIT: supposedly there's a patch coming soon to help address this issue.