Whats wrong with my PC? Can't run SC2

Hello,
Guys please help!

My computer:
Windows 7-32 bit
Asus Sabertooth X58
Intel Core i7 657 @3.07 Ghz
HDD: Seagate Barracuda 7200, 1 TB
AMD RAdeon HD 6950
Total P. Memory: 3063 MB RAM
Total A. Memory: 2323 MB RAM
Memory Devices: 2048 MB, 1066 Mhz (x3)
Thermaltake Litepower 700, ATX 12V, 2.3
Fan: Thermaltake Frio (I bought this because I thought cpu is getting overheated during the game..)

My problem is a black screen which appears (no signal) on the screen each time during the multiplayer game.. It is so annoying..   What could be the reason for that? Even though I know very little about system requirements, I know my pc is quite enough for a game like SC2.

Please help me.
Thanks..