The only way I'd be able to get a bigger battery it to relocate to the trunk. I guess you haven't seen a Civic up close, but the battery is tiny. While I had my sub amp hooked up, my volts would sit at 14.4 at idle, no music playing, and about 13.2 almost full tilt while in the 2000 to 5000 rpm range.
Counting both amps, my Kenwood component amp pulls 28 amps, according to ampguts, and I'm not sure on my sub amp, but it has 60 amps of fuses.
So that's around 88 amps of current, on a ~75 amp alt, and a tiny battery with a CCA of 500. I was seeing minimal voltage drops on 800 rms, but I'm not sure just how much more it can support without getting a H.O. alt.