Hi tmcd35,
What I prefer todo is stanardise as much as possible, Here at my school we all have Windows XP and a mix of Office 2003 and 2007, what i plan todo is upgrade all too Office 2007.
In your case I would hang on for Windows 7 and push that out, I say keep up to date with the latest software.
But make sure you have enough power on the desktops to run Windows 7 if you do go over to that.
Ian.