The only issue I've been having with Black Ops multiplayer is that my game will freeze for about 10-15 seconds every once in a while. I almost always get killed when this happens. I did edit my config_mp and lower a few graphical settings in-game, but it still locks up. I'm not sure if this still helps at all anymore, but I set my steam.exe priority to low and blackopsmp.exe priority to high.
Yesterday I hopped into a free for all game and I had terrible lag/stutter. I restarted the game and it was fine after that except for the lock ups.
Worst problems I had with single player was the cut-scenes were stuttering, but the patch mostly fixed that. Then there was the time I was playing through the campaign on Veteran, got all the way to near the end of the Rebirth mission and had an error which reset my campaign progress. It sent me all the way back to the SOG mission. Luckily I still had my save game files and was able to edit my config to get back to the Rebirth mission, but I had to start over from the beginning of the mission when I had almost beat it.
I have an Intel e8400 @ 3.6 GHz, an EVGA GTX470 SC+, 4GB DDR2 800, X-Fi Fatal1ty FPS sound card on Windows 7 64-bit.