It is true that Microsoft no longer provides support for OEM copies of Windows. It wasn't that long ago that they made that change.
The truth is that Win2k Pro and XP Pro are virtually identical operating systems (aside from the GUI) at their heart. There are just a few noteworthy differences:
1. The user interface is different in XP.
2. XP has some extra software utilities bundled with it. (firewall, movie maker, media player XP, can make a dos boot disk, file and settings transfer wizard, etc.)
3. XP has built in .zip file support.
4. XPs compatibility layer for older dos and Windows 98 programs is marginally better than 2k.
5. XP has the system restore utility, which can actually be useful.
On the downside, XP is a security nightmare, it is absolutely full of security holes. Unfortunately, some of that is Internet Explorer 6 related so in that regard 2k is the same. With the Home version of XP 99% of the systems I see have open guest accounts with blank passwords. (Hopefully, Service Pack 2 will resolve a couple of these issues.) It also has some USB issues that don't seem to be present in 2K. XP also has the very annoying product activation functionality, which can be very annoying for people who like to make regular hardware changes to their systems. (Let alone the fact that it allows Microsoft the legal right to stop activating copies of XP beyond a certain date. Fortunately, this will probably be no sooner than 2006.)
As for XP Pro vs XP Home, their isn't really much difference:
http://www.microsoft.com/windowsxp/pro/howtobuy/choosing2.aspMy opinion is that if you already have 2k Pro, there's no pressing reason to move to XP. (It's worth noting that 2k Pro users MUST upgrade to XP Pro. The XP Home upgrade isn't valid for 2k Pro.) For people choosing between the two moving up from 98 or Me, either 2K or XP will do nicely. 2k Pro and XP Pro are both about $175 for an OEM copy. XP Home is about $110 for an OEM copy.