Hello Serpentine, welcome to the tech forum!
Interesting that you have asked that question, as there is a current raging debate about it Rogue Nines thread regarding his new system
I use Vista x64 on my main gaming/work pc as well as on two dedicated home theater pcs.
Rather than issue you with a blanket statement that "x64 is faster" or "dont get it it sux!". here are a few of facts, followed by some important questions so we can fully address you original query
*Vista x64 bit contains an x86 emulation layer that means it can run many x86 processes. Mainly in instances where the app makes significant changes to the registry, is there an actual need for an x64 specific app. This is due to the differences between x86 and x64 registry structure.
*Vista x64 allows support for over 4gb RAM up to a theoretical 128gb with vista ultimate/business. >>still, you will need to decide if this is relevant to you and what you plan on doing with your system
*There are indeed older games that may not work in x64 Vista, but in this instance it is likely they wont work in x86 vista as well(even using xp/2000 compatibility mode). If you are really keen on games like KOTOR, The Monkey Island series etc - then Vista isnt a good idea.
*what do you mainly use your pc for ??
*what apps do you often use ??
*what games do you commonly play ??
*what type of hardware are you planning for your new pc, including max resolution of the monitor you will be using ??