There are games @ 1680x1050 on 9600gt that I am just barely where Im comfortable at in FPS (Hellgate for instance) and I dont play the insane gfx req games like crysis. 9600gt SLI would be AWESOME at this res. Even 8800gt sli wouldnt be overkill, but I doubt it's worth the extra $100+. I plan to buy a sli board this summer and then grab a second 9600gt when I feel the need.
Frankly, I wouldnt buy 9600gt SLI over 8800gt if you're looking for longevity, for the simple fact that if you buy 9600gt sli, that's it. No upgrade past that, but single 8800gt can be upgraded to sli later... of course if you upgrade every year or so, 9600gt sli is an amazing price/performance point right now. (edit: apparently 9xxx got some kind of sli tweak to improve performance too, but I havent really looked into this, Im sure there's 9600gt vs 8800gt sli comparisons around if you want to see that... I believe I've even glimpsed one with the 8800gt's using 174 too, so you can rule out driver tweaks on the 9xxx's... I suspect if the difference from 8xxx to 9xxx sli were big, there'd be more hubbub about it though)
Basically my concept if you're spending upwards of $200 on gfx is this. It better play everything you play now @ max. That's really all you can ask for. GFX move so fast that two years from now, you'll be able to upgrade your top of the line $500 card for <$100 anyway, so buying a $500+ card for longevities sake strikes me as silly. Believe me, I did it. I went 6800gt to 6800ultra to 7800gt to 7800gtx... and ya know what. It was awesome having the fastest card available (when I got the gt's the ultra/gtx's were paper launched) for about 6-10months. But now that I look at it, I really didnt need a card that fast back then... and I could have saved money in the long run by just buying a card that'd run what I needed it to run at the time and upgrading more often (even though I got most of my money back for the first few cards since I upgraded so close together). Buying a card for $150 every 8 months and selling it for $75 is cheaper than buying a $400 card every two years and selling it for $50. Plus if you're an overclocker, it means a new toy that much more often.