Vista, I think is now consigned to go the way of ME,
differences between vista and 7 are most that windows 7 runs a little slicker, requires a little less hardware performance, and for laptops has better power utilisation, meaning that your battery should last longer.
differences between xp and windows 7,
well xp is NT5.1 7 is NT6.1, so it is fair to think about it as the same kind of jump that there was in functionality look and style as there was between windows 98 and windows xp.
as for software compatibility, windows 7 does have a backwards compatibility mode, (just like windows XP has). but there is a second sort of compatibility mode for running apps that will only run in XP and that's to run XP on your windows 7 box as a virtual machine, (so you can have XP running on top of 7).
When you do this I'm told (according to the live meeting overview stuff that I'd attended over the past couple of days) that you can drag and drop between the desktops etc.
I really can't think of any situation where you're going to be running applications that there isn't an 7 equivalent of, (because Vista has been out for a couple of years and any application working on v6 of the kernel should still work in windows 7, which is still version 6).
but if you have some kind of bespoke business app or something then you can always run windows 7 as your man desktop and the run xp in a virtual machine on top of that.