I've been reading (and contributing to) this thread with interest. Here's my take on the whole thing:
Early SLI adapters saw almost no performance gains in SLI. It really WAS just a marketing ploy at that point. And why were there no performace gains? Because dual PCIe slot motherboards could only support the slots at x8/x8 (or if you were lucky x16/x8) vs x16 for a single card. In SLI, theoretically at x8/x8, there should have actually been a performance LOSS due to the master card having to consolidate the output of the dual cards, but the memory gain offset that allowing for a modest performance gain (either that or the x16/x8 configuration seem consistant with the 10-15% performance gain you often hear quoted).
Now, NVIDEA claims a 1.9x performance gain in SLI and numerous benchmarks that I've seen while researching over the past few months seem to bear that out (although not in all cases). The full realization of SLI is due to the advent of motherboards capable of supporting x16/x16 PCIe slots as well as the advance from split-frame to alternate-frame rendering. You still can't realize full 2x performance though (and likely never will) due to the master card having to consolidate output.
The advantages of dual proccessers can be seen in many computer technologies today; the ATI dual proccesser cards (2 proccessers on a single card... very expensive), dual core and quad core CPU's, Crossfire and SLI supported motherboards, PSU's, RAM, etc. There's no doubt that technology is marching toward multi-core proccessing, both in the CPU and GPU environments. I doubt it will be long before there are dual and quad core GPU's on the market.
In real terms, I've seen benchmark tests where both the 320 mb 8800 GTS and the 528 mb 8800 GT in SLI mode beat the 8800 GTX, but either of those options is more expensive than the GTX so the single GTX still provides the best bang for the buck. Furthermore, there is a real limit as to how much proccessing you actually need to run todays games on todays monitors, and the 8800 GTX will certainly run them all so do you really need more? In fact, a single 8800 GT or 8800 GTS will run almost anything thrown at it and the cost savings over a GTX or SLI configuration is significant. This tells me that from a pure performance standpoint, SLI is not worthwhile.
Still, I'm personally attracted to the idea of SLI both from the dual proccesser technology standpoint and from the standpoint of a "stop-gap" or "poor mans" upgrade option (outlined in a prior post). There's something to be said for just adding a second card as an upgrade path rather than throwing away your initial investment entirely.
So, in conclusion, both MrRiplEy and alaskahawk are both right... and both wrong. There are real gains to be had but at what price?
I'm ordering my new system tomorrow and, while I'll probably never use the capability I am ordering an SLI capable motherboard. I just like the options it allows (and I want the PCIe 2.0 slots... and as long as there are no single slot 2.0 boards out there then it may as well support SLI).
[EDIT] One more thing to add to this. I've read that the Intel X38 chipset fully supports SLI but NVIDEA has not licensed the technology to Intel to market the chip as SLI supported for fear of losing their own motherboard chipset business. That tells me that the offer from Intel simply hasn't been lucrative enough... yet, and that both sides are holding out in the negotiations. Once NVIDEA does license SLI to Intel, I think SLI will become a much more mainstream consideration almost overnight.
