The battery isn't really going to be a dependant factor, while running a car the alternator is charging the battery at the same time. If you are installing a large soundsystem, which I don't think you are installing a 'large' one, the stock battery is fine. Just take it to an auto-elec and get the alterator checked, and all wiring. Something might be loose.
For reference;
I'm running two amps, one 4*150 rms watt, and one 2*60 watt, in my 1986 Corolla AE82, with stock everything. Never gone flat. Regularly play it too loud with bass. The only thing that's wrong is that I didn't upgrade the big-3, so the headlights dim on the larger bass hits.
A large system is an amp with 2000rms for the sub, plus possibly 200 each side.