P4 or AMD 64 bit - sorry there is no clear answer on that one. It is AMD for me - as I think you get more for your money - but it is a difficult choice to make.
On your P/C the choices that I can see are:-
1) build yourself a new p/c and just swop over the bits from your current p/c that are still useable
2) bit the bullet and buy a new p/c (without the extras)