Hi there,
I hestitated for a long time before switching to x64.
I'm still using SW2005, so there is no 64bit version available, but using sp4 and up you can run it on x64.
I have 'only' 4 Gb of Ram, but in x64 SW can use ALL of it!
So the main advantage is a better memory management of Windows. I now can open my largest assemblies (>10000 parts) without any problems and even make drawings and section views without a problem! This was absolutely impossible on XP pro machines with 4 Gb (and yes, 3Gb switch turned on...). System would choke at 2.5 Gb memory in use.
PS: be aware that not all software will run properly on x64.
But office 2000 and solidworks 2005 run fine. Most of the problems I encountered with finding proper graphic drivers and printer drivers. But definintely worth the trouble !!!