Save $130 and get the Epox 9NPA+ Ultra. If you have no budget limits and want to game at very high res with high detail, then SLI is an OK option. If you plan on upgrading, then by the time you want to upgrade, the better option will most likely be getting a next generation vid card. A next gen vid card will likely cost less and perform better than SLI from previous gen - look at reviews and compare 6800GT SLI to 7800GT.
That first reply is RIGHT ON the money. The SLI thing just doesn't make sense.
If you buy two 7800GT cards for say $600 in SLI you will have a nice rig. But in 6 months when the next generation card comes out a single 8800GT or whatever will beat the two 7800GTs in SLI plus have the latest shaders and other technology the 7800s don't have. So SAVE your money, buy a $300 card now and then in 6-12 months buy another $300 card for the same price and get MORE for your money.
Unless you're a bloody rich SOB then GO FOR IT! And buy me something please....