Question: Whats better (2) GeForce 6800's connected via SLI or (1) Geforce 7800GTX? 

Answer: In general the 7800GTX is better. It will usually perform favorably when compared to a 6800 SLI setup and uses much much less power. For detailed comparison, see http://www.tomshardware.com/2005/12/02/vga_charts_viii/page5.html
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