(1) -march=pentium4 should be safe for your system.
(2) I had a box at work that had a P4 HT processor. I was surprised at how good of a job it did at acting like a twin processor system. You could go with MAKEOPTS="-j3", and get better performance from compiling. You may also want to keep MAKEOPTS="-j2" and have a slightly more responsive system when you are emerge'ing and doing other things at the same time.
(3) Changing CHOST is an advanced topic. You need to do a lot of homework before you start playing with CHOST. Save that one for later.