Overall, Win 7 integrates well.
Depends on how much you want to spend and how fast you need to be. Last year, I updated my average 1200-1600 Mhz machines to Pentium 2.4-2.8, all used and all had XP. The speed is fine. I do have faster computers at home and there is not a radical difference in speed. I use Win 7 at home. What about your other programs you need, are they Win 7 compatable?
So it depends. You can get refurbs cheap, but they are used and may not last as long. Or you can get new, with Win 7, which will last longer, but cost more money. If you are using a domain you will need the pro version.
As an aside, I discovered that V5 is NOT COMPLETELY COMPATABLE with Windows 2000. It worked on one machine, it didn't on another. Tech support said it is not completely compatable. Time to move on. Anyone else out there with a Windows 2K machine???