If you think you'll be wating 1+ years before doing so, don't. It's guaranteed to be cheaper to upgrade the card to one that'll outperform SLI.
And in that case i'd recommend ASUS P5K-E Wifi. And G92 8800GTS 512mb is the same as 9800GTX but cheaper. Also make sure you have a quality PSU, preferably one that has a Single very powerful 12v rail.
Tweaks the cause it to surpass the 8800gts barely in about half the benchmarks. Which means it's way overpriced. Unless you can find the 9800gtx for the same price, or cheaper, as the 8800gts 512mb then i'd never buy it.