2-3-4 GPU was mean to a PC that have more than one GPU card, multi GPU card in common words. In NVIDIA called
Scaleable Link Interface (SLI) while in ATI called CrossFire (CF). People have many reasons to go multi GPU, and many says to improve his performance with low budget. Why they call it low budget ? Because when they have low cost PC with an HD 5670, and have a little budget, they just have to put another HD 5670 then buy other expensive card. But there is a down side here, example a 2 GPU mode. Its not like have a 200% performance. If with 1 GPU got 100 FPS in game, with 2 GPU just about 150-160 FPS. Some even says they got the feel of microstutering. Microstutering is a micro laging cause by diferent output GPU. But its rarely right now.
With technology today, a microstutering almost gone. And the scalling of multi GPU is better, some can do a up to 100%. Theres only little multi GPU setup that have good scalling. A low cost HD 5xxx series, HD 6xxx series, and GTX 460 series. I can only recomending CF HD 6850 and CF HD 6870. They got best scalling in many games. Not only they won the battle with their competitor the GTX 460 in SLI mode, the CF HD 6870 even leaving GTX 470 in SLI mode. Here are their CF mode from guru3d.com :
But if you really need gameplay at extreme resolution, I recomended a GTX 480 for SLI up to 4 card. Here are some picture from guru3D.com, a four-way SLI 480 GTX :
Genta3D
