Yeah but then you won't have 470s in SLI for all that time
I would only go GF100 SLI if you were planning to water cool. Heat can be an issue with these cards, and I'd imagine your top card won't clock very well because of it.
The problem is your card is only going to sell for about $180-$200 right now, and there is nothing you can buy for less than $300 new that is going to beat it outside maybe the 6950 unlocked.
The other problem is even if you went to say a 570 for $300+ the performance different between it and your 470 (assuming it clocks well) isn't going to be worth the cost difference imo. That leaves the 580 which is an awesome card, with an incredible price tag.
Personally I'd get a 470 now, then start reading up on water cooling as it can turn these GF100 cards into different beasts with awesome temps and very high overclocks.
A custom GPU only loop would probably cost $320 or so I'd say for something decent.