IMHO, NO
They say that it is a good "upgrade path" because you can buy one 6800GT now and then in about 6 months, buy ANOTHER 6800GT and THEORETICALLY, have 2x the performance (we all know it is NOT 2x)
however, in 6 months, R520 and NV48/50 (dunno which one will come out) will be out . . . it is KNOWN that R520 will have 24 pixel pipelines, 8 more than the x800XT and 6800GT/Ultra
though they will be expensive, all you have to do is sell your current 6800GT on ebay or forum and use that money to get a newer video card . . . you will get better performance
One current example is with the 6600GT and 6800GT
6600GT is about $200 each while the 6800GT SHOULD be around $400-450
however, a SINGLE 6800GT CRUSHES DUAL 6600GT in SLi . . . why? The 6600GT has only 8 pixel pipelines, 128-bit bus, and 128MB memory while the 6800GT has double ALL OF THOSE compacted into one card
also, something that HURTS SLi . . . you have to use the same exact type of card FROM THE SAME MANUFACTURER
meaning you can't add a 6600GT to a 6800GT to make it faster
or if you buy, say, an XFX 6800GT and then 3 months later, you see a GREAT DEAL on a BFG 6800GT . . . sorry no can do, you have to get another XFX 6800GT
the ONLY people who would benefit from SLi are those with a TON OF $$$ to spend RIGHT NOW . . . and even then, i have heard things where people will install one card and get "X" FPS and then install another card and put them in SLi and get "X-10" FPS