like I said RAM. Large applications (DBs, web servers etc...) have needed more than 4GB of RAM for a while now.
Infact, Sun has been selling UltraSparc 64bit CPUs for a few years already.
The move for 64bit on the desktop is purely the smartest advertising AMD has ever come up with.
There is a performance advantage for 64bit. Some applications which use precise arithmetic like having the extra precision. Also I've heard some smart people can make memory access faster since twice as much stuff is moving per read/write cycle (not sure how that works tho!) Hopefully I'll have a better understanding once I've finished my compiler design class...