ok, so how about doing somthing like gtx 570 and 580, would that work? or does it have to be exactly the same model? Also, why exact same brand? What could be some consiquences of not having the same brand?
No the cards need to be at least the same series 2 xGTX460s or 2 x GTX 570s and it is better if they are the same exact brand and model.
Not really but they have to have the same core and have matching ID in the driver for sli. Example a 65nm 9800gtx is able to sli with a GTS250 due to the fact they have the same core and the same instruction set even if a new bios is needed. Something more modern would be a GTX 460 being able to sli with a SE. Asymmetric sli isn't very common but in practice it does pan out fairly well but scaling isn't going to be as great.
Yes it does! It has to have the same core and the same model as for what I had said was for only experienced users. You are clearly not. If the two cards do not have the same core it will not work period. Even the manufacturers have tried and fail in their labs. The gpu core has to be the common factor. Example G92 in the 8800gt,gs,gts, 9800gt, 9800gtx, 9600gso,9800gx2, gts250, gts240. They all have the same gpu but not all are exactilly the same such as disabled shaders and spanning different generations. Sli usually would work but to a experienced hacker since they have the same core and instruction set all that needs to be done is a few changes in the bios to enable sli. Move on to something more modern was the GTX260. The orginal featured 192 shader on a 65nm die then ATI put more pressure and Nividia released a 216shader model using the same core. In bios and in driver they had the same Device ID and are able to sli. Fast forward they later introduced the 55nm version of the GT200 core and a 216 shader model was of the first cards introduced with the same name and ID that allowed sli with the original. I could go on but I guess this is way over your head and the same for much of the people here.
nforce4max, I take offence to your assumption that I am not experienced. I am indeed new to Nvidia, and dont know the first thing about their cards, but I can assure you, friend, that I am not as unexperienced as you would like to think.