How do I improve the performance of StarCraft II?

StarCraft 2 recommends the "Ultra" graphics setting, but even on "Low" the FPS drops extremely low when there are a lot of units on the screen.

I've already downloaded and installed the latest drivers for my motherboard and video card and followed the steps in this performance optimization guide. Is there anything else I can do to make StarCraft run better, aside from upgrading my hardware?

The simple answer, very little.

You can kill all other running programs (virus scanners, services you don't need, applications like MSN Messenger, Skype, etc). Aside from minimalizing what is currently running on your computer therefor freeing up resources to be consumed by SC2, upgrading hardware is your next step. (You could try over-clocking, but that's mostly for the advanced computer users).

When you start to add individual units and the processing behind each and every unit it all adds up very quickly. The custom map Desert Strike is a prime example of this if it goes to the end battle where there is literally almost 1000 units moving towards each other. The computer's ability to handle a large majority of units and their animations/AI is a combination of all of your above components. You might find a light improvement by upgrading more RAM and a better video card.

**Side note: When you say you turned everything to low does that include death animations, shadows, ground clutter, etc.?

I think that CPU and Video is more than enough for SC2. I have a similiar CPU and game works like a charm.

Change sequence is: MB + RAM -> CPU -> Video

If driver updates cannot help, then I suggest you change MB to AM3 socket with DDR3 RAM. This will boost your performance more than a CPU and Video upgrade.