I didn't trust Vista's rating index and installed PC Mark 05 as it was suggested by a friend. Here's what I got:
Vista Ultimate RTM x86 5096 PC Marks with 3.5GB RAM
Vista Ultimate SP1 x64 4731 PC Marks with 4.00GB RAM
so obviously x86 is ahead of the game when using the same hardware. I will have to install SP1 on my main partition and see how that compares.
EDIT: I just installed SP1 on my main partition with x86 and I got a score of 5208 PC Marks with 4.00GB RAM
that's a difference of 477 PC Marks more for the x86, so I think I will stick with this instead of the x64