Galaxy GTX 460 2-Way SLI Problem

When i play this game called Mount and Blade Warband if i turn the anti-aliasing on it lags like crazy, i've just got a fresh new pair after rma'ing my old ones, i thought that would fix the problem and the other various problems that my old ones had. While i was waiting for new cards i used my old Radeon 4890 and the problem was gone. Why am i lagging when i turn on anti-aliasing?