If you had multisampling set above 1x I can only imagine that you're attempting to box at Ultra-like settings across all accounts, which isn't even possible. I don't even single box with multisampling set above 1x, and I run a mediocre system. People around here multibox with i7 processors and don't set their settings that high, it's not feasible if you're running multiple games on one machine.

It sounds like you've been successful at turning down your video settings and getting better results [not a surprise]. I would only suggest that you continue to do just that. When you hover over a video option you can indeed see whether it greatly impacts performance or not. Reduced Input Lag, Shadows, Texture Filtering, Multisampling... turn all of that shit down. View Distance also greatly affects your FPS.

Only you can find the right video settings that will make you happy on your machine.