I would have thought when playing multiplayer most people will be turning down the shader settings at the very least... at 1280x1024 with low graphics settings over 100fps average will be easily achievable on both cards with no major worries about min FPS. Crysis does SCALE very well, with just about any card you have to sacrifice image quality to get performance at the moment. With multiplayer being a competetive game the image quality will have to take the back seat for a while till hardware catches up with the game.
Neither card is any use for running crysis in multiplayer mode with graphics maxed... The graphics card for doing that will be released in the run up to christmas next year I should think
(thats pure guess work not based on any rumour at all.)