Nice thread.
Well, as sean_skroht says, the lack of drivers for XP is not always as real as advertised. You have to dig to find them, that's true. But the situation will only get worse in the immediate future, that's also true.
The good news are we are going to have CRT Emudriver for Windows 7 (Catalyst 13.1). It is going to happen within this year. This is not vaporware, a few users here have been testing the beta version. I spent several weeks around April-May of this year resolving the basic problems. I haven't released it yet because VMMaker is terribly outdated and I wanted to create a new version for the new package in sync with the "new" GroovyMAME crt_range format, but free time is a rare treasure for me these days.
So there's no reason to stock pile XP compatible motherboards or anything like that. By next year most of us will be using Windows 7.
The real bottle neck is actually on the video card side. Nothing newer than the HD 4xxx family works. I've tried everything. No go. Fortunately the second hand market for video cards is quite healthy. So we have fuel for some time until the PCI-e port gets replaced by something else.
Unlocking the newer ATI cards for 15 kHz (HD 5xxx, HD 6xxx, HD 7xxx) would be a great achievement, but it could be just impossible. The only strategy I can think of involves learning how to do live debugging of the drivers (kernel debugging), and maybe just to find it's not possible.
Besides, at some point, video cards will loose their analog outputs.
Windows 7 is still much more complicated to get properly configured for 15 kHz than Windows XP. It's not that it's more complicated actually, it's just that many more things can fail. And if you see many of users getting stuck installing drivers in XP, I don't want to imagine when they have to care about enabling test signing, UAC, etc.
Finally, it's a shame we have to abandon XP, now that things were getting really interesting

At the time I was experimenting with W7, I also got a new beta of the driver for XP, that allows you to create new modes *without restarting*. This makes VMMaker unnecesary! No need to have 100+ resolutions available. Only one for the desktop. You just need to install the driver and run GroovyMAME, change the monitor type and new modes would be created as required, just like we do in Linux.