I dunno about hurting performance... unless one module can't handle the speed you're running them at, and it creates errors that the CPU must take time to correct. But running two sticks together period can limit what your max RAM overclock can be, and how low you can get the timing... If one stick won't do it, neither will. And the nForce2 has some issues with dual memory channels... some memory is incompatible with it when running in dual channel mode.