in "configuring the compile options" of the gentoo install guide.  I was confused about what to set -march to for the CFLAGS variable.  arcoding to kinfocenter my processor is "Intel(R) Pentium 4 CPU 2.20GHZ".  acording to make.conf.example

# CPU types supported by gcc version:
# (Higher versions include the targets from older ones,
# those listed on the same line are equivalent.)
...
...
# pentium4              Intel Pentium4 with MMX, SSE and SSE2.
...
...
# prescott              Intel Pentium4 with MMX, SSE, SSE2 and SSE3.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

so should i use some thing like:
CFLAGS="-march=pentium4 -O2 -pipe"
or
CFLAGS="-march=prescott -O2 -pipe"

I guess my question is what is the difference between pentium4 and prescott?