Thats just the thing, why would a piece of gaming equipment - especially if you're a serious gamer - last you 2 or 3 years? Hardware evolves so much faster than that, and software evolves to require its demands. I think an 8800 would last you a year tops, that is if you wanted to keep that same high end performance.
If you buy something thats running at 80-90% of top end for a third of the price, you can afford to buy an upgrade much, much sooner. If its a third of the price (or I should say value), then you can upgrade it twice using the same amount of cash as an 8800.
An 8800 or a 7900 and two future upgrades? I'd pick the latter - if I was on any kind of a budget, that is.
edit: take into account price fluctuations as well - how much is an 8800 going to be in 6 months time? if it really would last you 2 or 3 years, just buy a 7900 now and upgrade to an 8800 in 2 or 3 months if you really want to - same result, probably far cheaper.