You will need an SLI enable motherboard.
I believe Asus have one out at the moment. Check it out on their website.
As long as the motherboard supports the CPU then there shouldn't be any compatability problems.
I am not sure if you can use cards from two different manufacturers, best to check on this.
Also you can just run one Nvidia card initially then when that one begins to struggle you can by another card...hopefully cheaper because of depreciation and just pop that in to give ur PC a bit more zest.
SLI is an Nvidia only technology and yes is only available on Nvidia cards that support the technology (6x00 series I think).
ATI are working on a similar technology that apparently doesn't need the little bridging PCB that connects the two cards together. This will prob be available mid 2005.
Any more Qs I'd be glad to try and answer them.