YOU are the one who bragged about getting a 900 watt amp and a couple of subwoofers and listed NOTHING else in your "system", not I. There is a whole other slew of insults I can throw your way, but I will be nice for the time being!//content.invisioncic.com/y282845/emoticons/tongue.gif.6130eb82179565f6db8d26d6001dcd24.gif
Regardless, I will attempt to help you answer your question. Sadly, it all depends on your situation and how sensible you are with the volume knob as to if your stock battery will work for you. What is the AH discharge figure on your stock battery? What is the maximum amperage draw from your proposed amplifier? Do you plan to listen to your subs at full tilt all the time?
Worst case, try your stock battery and see if it is enough or not. If it is, you are out nothing, if it isn't, then you can look into upgrading.