kea0463

1) Update make.conf:
# emerge app-portage/cpuinfo2cpuflags Run:
# cpuinfo2cpuflags Then update your CPU_FLAGS_X86 (cause I think that there are also mmxext (native in Athlon but renamed in Intel's cpu, such as sse3 are named PNI ("Prescott new Instructions))
 
Then You're core2 is precedent of "Penryn" family that supports also sse4.1
 
If you want try also 
grep sse4 /proc/cpuinfo to be sure that there aren't also sse4.1 streaming simd extensions.
 
For Other questions, mmm I use the "Stable branch" of portage.
 
Ciao :)