Vista doesnt need 4GB of RAM. I run Vista just fine on a P4 and have run it on 1.5GB of RAM just fine for internet and documents. That is a farce that you need 4GB of RAM to run Vista smoothly.
Not to mention that XP is no longer for sale via OEM. The only one you can buy legally now is the Retail version which goes for about the same as Vista OEM if not more.
Your right, Microsoft wont be having the newer software be compatible with XP. As Vista is the new OS and Win7 will be out before the holiday's. So there is no reason for Microsoft to allow XP to be compatible anymore. With XP Mode in Win7 even their older apps can still work just fine with allowing them to not make the software work on XP.
If you really want, build the machine now and use Win7 RC which is good till June of 2010. Then buy Win7 at that time.
So either way you are going to spend over $100 to buy a OS. XP Retail is still that high and Vista OEM is that low. the choice is yours. But if you want to be ready for the future with this machine, XP is not the way to go.