* Cards from two separate retail companies will work together in SLI mode, but they must be the same GPU model (e.g. G70, G73, G80, etc). The cards may have different BIOS revisions, different default clock speeds, or even different memory sizes. However, the fastest card - or the card with more memory - will run at the speed of the slower card or disable its additional memory.
* SLI doesn't always give a performance benefit — in some extreme cases, it can lower the framerate due to the particulars of an application's coding.[2] This is also true for ATI's CrossFire, as the problem is inherent in multi-GPU systems.