Capacitor or any other mods?

ok i have a 98' cavalier and am puting in two 12" kenwoods whos RMS is around 300-500 i think...they max at 1,200 and then a 1,200 watt amp so figure ill have about 300 watts running to each speaker so 600 total watts running. plus the regular speakers. iv got the proper amp hookup and harness for my stereo im putting in but do i need to get a capacitor or better alternator or should i be good?