If you are worried about bandwidth on x8 PCIe slots, why are you considering 9800GT SLI? ;) Seriously though, the x8 bandwidth is not an issue and results in very little performance degradation.
A 5870 actually. I purchased mine for $420, double lifetime warranty and all. The 5970 is two 5870s in internal Crossfire set at 5850 clock speeds and regularly retails for $650-750.
The ability to use 2 monitors on a single card has been a feature for quite awhile; I was using two of my new monitors on my GTX260 up until about 2 weeks ago. The ATI 5xxx series allows you to use up to 3 monitors on a single card; a feature typically reserved for workstation cards. The best feature (in my mind) is that you can use those 3 monitors for triple-monitor gaming (known as Eyefinity). I've been having lots of fun using it so far. :D