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Just upgraded to gtx sli, I run games at 1920x1200 with 2-16x aa/16x af, have to say I was blown away at how fast it was.
If you have a 24"+ screen go for it imo.
As some have already said - depends on your screensize. I have 2x 8800GTs but only a 17" 1280x1024 native so decided to sell one of them as it's dead money sat in my PC.
5.46 Avr FPS difference in Crysis on high, 0x AA when I tested it. Don't really see the benfit of SLI unless you have a big monitor or play with high AA levels, which I don't.
But the Card(s) are coming to 1+years old so I would not add to it and wait and see whats new in Feb 2008.