What are the advantages / disadvantages of choosing a 64 bit processor when buying a new PC?

I know that they support more memory, and I think they require a special OS (?), but that's about it.

Thanks!

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Basically, speed and greater task handling. They're generally used in Server type applications, but many home systems today are gearing up with them, mostly gamers and video/graphics editing orientated people.

They are "backwards compatable" in that they will handle 32 bit O/S's and applications. (Kind of a waste though to install a 32 bit O/S on a 64 bit system.)