|Unless for some reason the game requires tons of upstream bandwidth (games usually only care about downstream bandwidth), a fast AGP card should run SC2 fine. |
You should aim for something a little higher than the 6600. Every once in a while, TigerDirect offers an x1950Pro AGP for $64.99, which is a great deal.
Insanely 939 | Dual-core Opteron 185 @ 3.2GHz | 4GB CL2 PC3200 | 1.0TB
A8N32-SLI Deluxe | Blu-Ray | X-Fi Ti Pro | NEW! GeForce GTX 460 1GB