Stop posting that there is a DX10 hack for XP, there is not. It makes your PC think it has DX10 but it does not, why would you even bother with it?
32x Vista SP1 (and not sure about XP SP3) fixed the issue with having 4gb of RAM appearing as 3.5gb, ect. So there is no reason to choose it for the extra RAM, cos you won't really go over that, will you?
IMO stick with what you know. XP has been around for a while and has served us well, many companies (nvidia for one) have sated that they will NOT be using Vista in their businesses (that is a loss of 80,000 licenses form nvidia alone not using it), but will stick with XP till the new OS is released. I personally will be doing the same unless i REALLY need it for DX10 in which case i would dual boot Vista for gaming ONLY.
But just bare in mind that i would never consider gaming on Vista without 4GB of ram.