Minimum:
Windows® XP/Vista/7 (latest service packs) with DX 9.0c
Intel Pentium® D 2.8 GHz or AMD Athlon™ 64 X2 4400+
NVIDIA® GeForce® 7800 GT or ATI Radeon™ X1950 Pro or better
1 GB RAM (XP), 1.5 GB (Vista/7)
12 GB available HD space
DVD-ROM (required for retail disc versions only)
Broadband** Internet connection
1024x768 minimum resolution

Recommended
Windows® Vista/7 (latest service packs)
Intel® Core 2 Duo 2.4 GHz or AMD Athlon™ 64 X2 5600+ 2.8 GHz
2 GB RAM
NVIDIA® GeForce® 260 or ATI Radeon™ HD 4870 or better

I've currently got a gtx 295. It has some lag playing sc2 on ultra when there are a lot of units. I need to upgrade video cards soon. 3 years old is practically ancient.