Dual booting XP and 7 isn't a problem.. however, to make it work right you have to make sure you install old to new.. As in Xp first than Windows 7.. and since you already have XP installed than your good to go... It's just a matter of installing 7 over top of Vista OR installing it on a whole seperate partition..
As for drivers.. unless you can find a Windows 7 specific driver for your hardware than I'd suggest using the most recent Windows Vista driver as they are supposed to work with 7..
Once 7 is finished (or before) there will be 7 specific drivers available but like I said for now you can most likely get away with using the most recent Vista one's..
.. this also applies to the SLI question..
I'm not 100% sure if...