Extra battery?

Yeah get a cap, strain your alternator more, good idea.  Anyway to get a rough calculation of how many amps you are going to need you need to divide your power into volts.  You want to take your rms wattage and double it, so (1800 x2 3600), so 3600 /14.4 ( I am only guessing 14.4, I don't know the actual voltage of your car), so you will be pulling roughly about 250 amps, but there are so many variables in this equation, just get somewhere in the area of 200 amp alternator if you can and you should be good.

alrite thanks alot, and with 1800 watts rms do you think i should get an extra yellow top battery? and your kidding about about the cap right.

I don't think he is kidding, but do not buy one. They ARE NOT an additional load on your alternator, but they do little to NOTHING for a system. If you have a yellow top battery already, yeah, another couldn't hurt, but if you are not using a yellow top already, GET AN IDENTICAL BATTERY TO WHAT YOU ARE PRESENTLY RUNNING. Run it in parallel with the first battery, and leave out any and all isolating devices. They do as much for a system as a cap does - NOTHING.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

alrite thanks you guys. and right now im using 4 guage to wire everything. when i get another 1200watts rms running what gauge should i wire it at

Well I was not kidding, I was being sarcastic.  Caps will not generate more current, only store it, sorry for the confusion.  You will probably need to run 1/0 ga to be on the safe side.