I've done a bit of googling and haven't turned up any useful information on this topic:

I've seen some info related to passing the -march=athlon flag to gcc for some performance enhancements (on the athlon architecture of course). Does anyone know what kind of performance benefits would come of this?

If there's significant advantage to be had by using it, I'd be interested in making it a default compiler flag (if there's a way to do that - anyone know?) and/or the default for rpm building (which apparently is hiding somewhere in /etc/rpmrc or /lib/rpm/rpmrc), since I'm probably not going to be compiling stuff for anything other than the CPU I'm running on.

If anyone has experience with this or knows a good source of additional information, I'd love to know more...

A good source for info about compiler flags in general is the gcc website at htp://gcc.gnu.org Just go to documentation and check with the doc's for the version you use.
Your best bet to find out what CPU flag is right for yours is probably a search at the gentoo forum at http://www.gentoo.org for 'gcc flags'.
It is a little messy with AMD's - I got a duron and there is a duron flag but I need to use athlon-tbird.Depends on the actual core of the CPU not the name.