keep in mind that if you are going to run WinXP64 bit edition, vegas will render slower than on the 32 bit edition .. but (so far) it seems to be running fairly well on winxp64 (except some issues with sql desktop server - not sure if 64bitXP related)
There is a .NET64 bit version which you should run instead of the 32bit version ..
DVDA doesnt burn in the 64bit version - but not really an issue ..
There are special 64bit and dual cpu versions of some codecs around that work really well (some free and some fairly expensive) .. Virtualdub has a 64 bit version, so does avisynth ..
see list i did of 64 bit applications in the 100% cpu usage thread:
XViD has a 64 bit version of thier codec, so does huffyuv and DiVX has a 64 bit beta version ...
I installed the 32bit (NET) , then removed it and installed the 64bit (NET) .. just to get vegas installed ..
Also instead of installing from the single exe file, installing from the seperate components from the expanded installation directory gives you more control (also can change install script)