I'm debating getting a SLI set up on my new computer right now. I'm either going to go with two GF 6600GT cards or just get the radeon x800. I'm not sure which way to go though.
From what I've read, Nvidia has to actually release drivers for each specific game in order for the SLi to be really utilized. There's only a handfull of games (maybe 15) listed on Nvidia's website that are currently utilizing it.
I've read about ATi Radeons plans to work on an SLi like setup in the near future. The other advantage, though still a rumour at this stage, is the GPUs won't need to be linked to each other. I'm sure it's called AMR.
Check out this link if you want SLi to work with ANY GPU using dual PCI-E for Intel processors:
www.tomshardware.com/hardnews/20050106_120044.html