140 is plenty for most people for daily driving. SPL guys, 145-150+ daily, but that WILL cause damage (to your ears, to your vehicle, and to your wallet (tickets //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif))
but to answer your question about an amp for those subs. i would do anything between 1500-2000 watts on the whole pair so 750-1000 on each sub. the Mclass amps are very nice. especially the 2500d if u could afford it. but i think the new Memphis MC1.1500 would be a great match for 2 dual 4 ohm m3 15s.
good luck with your build. and DONT use a capacitor.