ATI's SLI solution is only a rumor now and will only likely be released in the middle of this year sometime or even later so donÂ’t bother waiting for it to come out.
As for with one will last you longer I have to say the 2 x 6800 Ultras in SLI.
Although you cant get them yet in any stores in PCI-E versions i suggest you get the 6800GT's instead if you want it straight away it will be the exact same thing as the ultras.
Also make sure your mobo support SLI.
Neosteve