With gcc 4.3 or better, you should use these CFLAGS in /etc/make.conf:

CFLAGS="-O2 -pipe -march=core2 -msse4.1"

For older gccs try it with -O2 -pipe -march=-nocona

Since it is a dual-core processor, you should use three parallel make processes. In /etc/make.conf:

MAKEOPTS="-j3"