Like everything else, there are the diminishing returns you are going to get as time goes on. You don't mention the capacity of the machines but it is possible that you could reformat the drive and install windows 7, 32 bit and continue for quite a while longer. I have about 16 dell d620/630 notebooks and 2/3 of them are win 7 now. I bought them that way. I wanted to keep xp but have learned to deal with w7.
Drivers for printers and other devices, USB devices etc. will become harder to find for xp as makers just move on.
The good thing is that W8 seems to be on the way out too and Microsoft will no doubt be bringing out something akin to W7 phase 2 that will actually be friendly enough to work for our needs.