I too have the same problem with me being able to run at high settings on single player, but not even able to play multiplier on the lowest settings :( Some servers work fine but I've only found two.
My Specs:
Intel Core 2 Quad - Q8400 @ 2.66 GHz
nVIDIA 660 GTX - 2GB
4GB DDR3 Ram @ 1600 mhz