You have to keep checking Microsoft's page to see when they're going to end mainstream support for XP64.

http://support.microsoft.com/lifecycle/?p1=8599

Usually they give a couple of years before mainstream support ends.

As far as Vista goes, if you buy a computer it should have the option of burning the operating system to disk. However, not many computers seem to come with the 64 bit version of Vista. My laptop did not. I had to buy Vista Ultimate separately.