I've NEVER had a problem with an ATi driver. I don't know why people say the drivers suck. The only problem I’ve ever had with a graphics driver was nVidia, and that was trying to install it on my old Asus laptop. It would not let me used the official nVidia driver but forced me to use the outdated Asus one from a year ago. And that's not even really a problem with the driver itself but rather policy. I don't really think the driver debate is as relevant as it once was. If you are really conserned look for the current ATi/nVidia bug list to see if any current bugs affect you.
nVidia has some more powerful cards right now but they cost a lot more. I think it's always best (if you like to save money but love performance) to buy second gen stuff, the stuff right below top end.
For SLI you will need an SLI capable motherboard. Here is a capable SLI MB: (
http://www.newegg.com/Product/Produc...82E16813128339)
As for vista, I love Vista. It grows on you, I know a lot of people who hated it the first time around but learned to love it the second time around. Stay away from 64Bit for awhile. If you like to mess with things it's ok but it's really not primetime yet. I managed to get XP x64 to work a little over two years ago but there are still some driver manufactures that a slow or no longer make drivers. As for program support in Vista I’ve had no real issues. Sometimes you will come across an issue with an old game but that's usually just downloading a dll file off the internet and placing it in the main game folder. If a dll error comes up during runtime just search Google for it, download it and put it in the game folder and it should work no problem. I've had to do this with a few games but they all work.
If you can live with XP for another 1.5 years I would say wait because Windows Vienna is expected Q4 of 2009.