Apart from the 4GB RAM limit, the other main reason for my switching to 64-bit was simply... it's going to be the standard sooner or later. I just made a big spreadsheet of all my installed apps & drivers. Then I spent a good half a day on the interweb checking and downloading 64-bit versions or updates of the drivers, and noting any 64-bit incompatibilities with my apps. Burned them all to a DVD and did a back-up... then just did it!
Sure, there will be at least a couple of things down the track that you smack your head and realise, crap, that doesn't work. But I never encountered anything that couldn't be solved. My worst incompatibility was discovering that my aging digital point-n-click camera (that I forgot about in the preparation) didn't have a 64-bit driver. Good excuse to go get another, better one though
Andrew Solomon
Solid Architecture
AC12, 2675 AUS full Intel Core 2 Quad, 2.83 GHz, 4GB RAM, Vista Business 64bit sp2, ATI Radeon 4870 1MB