Question about Windows version

The Help Desk is still not recording new shows (yet), but here's a question from a listener. If you have an answer, please Comment:

I have a 5.5 year old home-built PC running XP Pro. In addition I have a Vista running laptop and a 6-month old iMac.

I still have a great use for a Windows machine but don't think I really want to build again or buy a ready-made box. Instead I'm thinking about using the iMac in a dual boot mode or using software like Parallels. The problem is that I don't know which MS OS makes the most sense.

XP, while older, is tried and true and should be supported for some time thanks to its use in netbooks. Plus, there would definitely be no comparability issues because all the software I currently run works on XP (obviously).

Vista, also a known entity, is newer and more secure, but doesn't bring much else to the table.

Windows 7 is an unknown quantity, but is what I would install were I to build a new machine, so it should clearly be in the running.

So what would be the best choice for dual booting on a Mac? Dollars and cents are in favor of XP and Vista, but 7 is the future.

What do you guys think?