Keep in mind that we are talking about 64 bit Operating Systems here.
Just about every CPU you can buy at moment is actually a 64 bit CPU, so almost every computer you buy will be 64 bit machine. If you can get it install both a 32 bit OS and a 64 bit OS on the machine. That way you have a choice and if you do encounter a program that will not behave in a 64 bit environment you can always fall back.
In a new machine with all new hardware 64 bit Vista should work. If you want to reuse old hardware 64 bit XP is probably a better idea. For the 32 bit OS always go for XP (proven technology).