How much power do I really need for subs?

Agreed with Ravendarat, and I think you should get some good mid-bass from those 6x9's.  As far as the original question, how much power for the subs,  I agree with kfr01 with the 600 amp.  And, importantly, do you have a 4 channel amp for those speakers in the car?  Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

I am going to get a 4 channel amp for the speakers in the car, You are right it needs it !

Good.  I am using a 4x50 watt amp (@ 4 ohm) and a 200 watt amp for the sub (@ 4 ohm), and the music gets as loud as I can take it.  And no upgrade to the alt or battery was necessary.  400 clean watts of power will give you a rockin' car.  You will have more power to your subs because they need more, but a 4x50 amp for speakers may well do what you want it to do.Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

I've installed killer sounding systems with as little as 35 watts for the sub and 15 watts for the mains.  It all depends on the components used and the desires and tastes of the owner.  I've seen loudspeakers routinely hit 110db on 10 watts input.  Decide what you want out of your sound system and pick the right components that sound good to you.  Don't get hung up on power ratings and big numbers as they are often sales devices and have nothing to do with how a system will sound.Support the12volt.com

and on the other hand of things... im in a band and used to things being extreamly loud... so 50 watts x4 is not quite enough i just upgraded to 90 wattsx4... but it all depends on how far you want to take youre system and how much money youre willing to spend....    and like they said, if you do spend some money and effort into it.. welcome to our hobby.. its fun

Thank All of you for your Help This Is a Good forum!