Upgrading to 64 bit from a 32 bit installation on FBSD 7

[Andrew Berry]

Hi,

I've got a server which will be upgraded to my "castoff" parts which  
support amd64. This machine has been running Freebsd since the 5.0  
RC's, starting on a P2 233Mhz, upgraded incrementally for a few years  
- a real testament to the stability of a FreeBSD install! What I've  
done in the past while upgrading is simply move the drive, and rebuild  
everything with an updated make.conf optimized for the new processor.  
Can I simply rebuild the world (or use freebsd-update), and  
portupgrade everything to rebuild for amd64? Or, will things break as  
libraries are upgraded which still have 32-bit dependancies on the  
system?

Also, AFAIK upgrading to a 64 bit system allows access to additional  
registers on the CPU, leading to a performance increase. The system  
will only have 1.5 gigs of RAM, so that's not an issue, but are there  
any benchmarks out there comparing performance on < 4GB hardware with  
32 bit and 64 bit Freebsd?

Thanks!
--Andrew