I have Vista 64bit ultimate on my desktop (installed on twin disks RAID 0). AVG seems to work fine, not given any grief as of yet, updates as soon as Vista is logged in etc and does a routine scan. I installed AVG on the first day I got the PC so can't really compare it to anything else.
I haven't tried uninstalling it as of yet. From past experience with my laptop it seemed to speed up a lot after removing McAfee and installing the free version of AVG (32bit OS).