Hello TonyGoo,
I run x64 Vista Ultimate and can recommend it, you will not have any trouble with run of the mill software but you may have a problem if you are thinking about running older/specialist software.
Driver support in general is good. I have never had a problem obtaining a driver.
6 Gig of Ram will give you an excellent experience with x64 Vista
It would have to be old hardware not to be supported by x64 Vista, if it's that old then best not to use it, lol.
You will always have people who have had a bad experience of x64 Vista and bad mouth it, or are very negative, but then, if you look back you will find that is true of all OS's.
Hope this helps, let us know how you get on, good luck.