I use Vista at home currently for my 5 boxing adventures, and honestly I have 0 issues with it. The very reason you mention is exactly the reason why I upgraded to it. You can stick with xp and do horizontal span if you like, but i hate that view mode as if you have just the two monitors it will split things in the middle across the two, which looks terrible in my opinion.

Really, after turning off the "are you sure, are you sure?" questions in Vista, I can even barely tell a difference between it and XP aside from the dual view fix. It looks a little prettier, has some snazzier behind the scenes functions (true symlinks anyone?) and also uses a bit more ram, but it sounds like you've got enough ram.

Lots of people hate on vista, but I have no issues with it, I'm very happy with my upgrade.