[OxLUG] Best way to upgrade to 64 bit - and is it worth it?

I have just finished upgrading my trusty home PC to new motherboard and
AMD64X2 CPU. After some kernel recompiles to recognize the changed SATA
hardware and the increased memory and CPU's, all is now running fine on
the code I compiled on the previous Athlon XP (32 bit) processor.

I'm using Gentoo by the way (I know, its not Debian, sorry).

I think this means that I have the opportunity to try and upgrade the
code I'm running to use 64 bit versions. I'm just really unclear about a
good way to go about doing that, and whether its worth doing - in terms
of either improved performance or features.

I'm really fuzzy on the whole 32 vs 64 bit thing. I assume that my
currently compiled code is aimed at a 32 bit processor, and the AMD64 is
OK with that because the 32 bit instructions are still there. Would it
be possible to change the Gentoo build settings (CFLAGS I guess) so that
I start compiling in 64 bit mode from now on? Or do I have to rebuild
the lot in one exercise - like when I had to change the gcc library
versions for example.

And the kernel - is there a special 64 bit version of it? If I change to
that do my 32 bit apps stop working?

Sorry this is all a bit basic. I've tried Googling for AMD32 to AMD64
upgrade, but nothing relevant appears. The Gentoo
documentation/wiki/forums all assume you are starting and installation
from scratch - given this system has evolved over 5 years I'm not keen
to try and start over obviously. I thought I'd ask here to see if people
have tried this sort of upgrade.

Thanks
Ian