[Clfs-support] BUILDFLAGS guidance

[bv]

Greetings,

I build CLFS for 64bit AMD64 (athlonX2 or Sempron cpu's).  The clfs book 
recommendeds setting -m64 as ${BUILD64} which I assume is  an integral part of  
${CFLAGS}:-
http://cross-lfs.org/view/svn/x86_64-64/cross-tools/flags.html

Most amd64 cpu come as a minimum of 2cpus (dual-core) nowadays. 
Trawling the internet I glean   safe ${CFLAGS} for 64-bit compilation  are 


CHOST="x86_64-pc-linux-gnu"
CFLAGS="-march=k8 -O2 -pipe"
CXXFLAGS="${CFLAGS}"

or 

CHOST="x86_64-pc-linux-gnu"
CFLAGS="-march=k8 -msse3 -O2 -pipe"
CXXFLAGS="${CFLAGS}"

If one was to set either of these (line2) as the ${BUILD64) to use for a 64-
bit CLFS build  would    -m64  be still needed  as an  append?

Or are  these  set automatically and I have whole thing muddled?

thanks in advance
bvol