XP or Vista and 32 or 64 bit

I am currently a user of Windows XP 32 bit. I am building a new system and debating whether or not to get Vista. I am also debating whether or not to get 32 or 64 bit. Here is my gaming scenario. I will be building a system to play Everquest and WoW only. No plans for any other games. Might be running two accounts at once maximum, and flipping back and forth to internet at same time. The system will consist of the following:

E8400 processor
8800GTS 512MB video card
4GB DDR2 800 or 1000 memory

My questions are:

1) Should i use XP or Vista

2) Would i benefit at all running these application under 64 bit with 4GB of Ram......or should I stick with 32 bit?

Thank you for any input

Vista 64bit

I just installed this OS and it was the easiest OS install ive ever had..Just gather all your 64bit driver's together before you start. I had it all up and running playing crysis in 2 hour's