If you are willing to consider Windows 7 64-bit then build a Vista 64-bit / Windows 7 64-bit certified machine (as best you can) and go with Vista 64-bit. Why?
1. It can use 4Gb of ram and more. XP Pro (32-bit) cannot
2. Vista is a better implementation of 64-bit than is XP Pro 64-bit
3. Vista can be tamed into Windows Classic and look and feel just like XP Pro.
4. Windows 7 is built upon Vista's structures. A number of reasonable experts place it as Vista upgraded.
5. No matter whether XP 64-bit, Vista-64bit or Windows 7 64-bit, you have to upgrade many of your applications. Legacy applications against a 64-bit OS is a strategic blunder in my opinion. If you think compliance issues will be solved with Windows 7, I don't think they will except insofar as vendors have more time to become compliant.
6. Again if you are willing to go Windows 7, you might as well learn on Vista platform.
7. Properly set up, Vista 64-bit is rock solid. I have no reliability problems with my daily driver (Thinkpad Vista 64-bit)
... Thinkpads_User