There are quite a few problems with the "buy one now, then later for a bost" idea. Don't get me wrong, its an awsome idea - I'm all for it - its just that its VERY early in the game for "gen-2" SLI's.
The main problem - price & depreciation. By the time you decide to get a second card, chances are for the price of a dual 16x PCIe moboard *AND* the 2nd vid card you could just get the even-next-gen 'x880 pro' or '6900 ultra' versions (tentative titles
). FYI the only dual 16x PCIe moboards are server boards, not quasi-server FX or EE boards.
[Option A: moboard $ + 2nd 'slightly outdated' gpu $]
[Option B: next step in vid card lineup]
I'm willing to bet that 'tomorrow's lineup' of cards will net you ~40% more FPS than today's & possibly more features in some way - all for about the same price for a mobo+old gpu. The main difference will be the fact you can sell your 'yesterday's gpu' on ebay for ~1/4 your 'tomorrow's gpu' purchase price.
If you see it as balls-to-the-wall today and get 2x cards... First off, no game can really stress ONE of these cards enough to warrant a 2nd. If you have close to $700+ for vid cards alone, chances are you're also running an FX/EE and don't even need that much gpu power. Chances are this same person will also have the money to update for the next revision of cards, which totally shoots the sails out from their "I need the power for *future* games" argument for these people.
IMO, the only people this can really benifit are individual users who don't have $1300 for Quadrios atm but need them. And last I heard, MS was working hard to get some distributed PC OS working soon, which would become the better option for this user.
From the Voodoo2 reviews I read long ago, the only normal user that will benifit will be Mac users...although new card models now migrate to Mac much faster than they did in the 90's.
edit: forgot to mention - in the past, the Voodoo 3's had virtually the same performance as dual Voodoo 2's, but for less...at the time the market saw the scaling in GPU power so great that dual card setups weren't viable.