I would say go for the 64bit option,
Im running xp x64 now, where as 4 months ago i admit i couldnt,
but now with all the drivers and such coming into shape i am loving it.
I do have 4gb of ram though, so 64bit is a god send as it can address all of it.
I have had no issues with programs or drivers in recent months.
And with vista being a new os there should be plenty of drivers which will be written for it.
From what i have heard most dev's didnt write xp 64bit drivers due to the fact the new os "Vista" was on the horizon.
You have a 64bit cpu, make use of it
If you want to try it out, you can download a xp 64bit trial for free from ms anyways. I dont have the link though sorry.