You will get an absolute ton of problems with drivers etc, once you get it working though X64 is fantastic, I would install it on mine but I wont bother changing till Vista comes out, my dual core processor is wasted no XP Home.
I think I did manage to get Norton installed on mine but it took some drive swapping to get it working...
The best other I could recommend would be Sophos (
www.sophos.com). Works well and isnt intrusive which is why I hate Norton.