I'm assuming you mean 2GB of ram. Vista 64 bit has absolutely appalling driver support (as has xp 64) plus very few programs run in 64bit mode. If you need loads of RAM then it's the way to go though just don't expect all your hardware/software to work with it.
I thought Windows XP could use 4GB of RAM (but only use 3GB or something)
Memory Support and Windows Operating Systems
Says something similar