I think the proper gcc -march flag for amd64 would be -march=k8. At least that is what I use in my /etc/make.conf for all my amd64 gentoo servers at work. My gentoo box that I run eqemu on is an old 1 Gig athlon-tbird. I make sure to change all of the makefiles to reflect this by switching -march=i686 to -march=athon-tbird and -O to -O2. The same settings that I use in make.conf. I also make sure to remove the -pg flags while I am in the makefiles. I have yet to have a compile error "knock no wood"...
Here is a helpful guide on the gentoo wiki about cflags
http://gentoo-wiki.com/Safe_Cflags