And go for the good ones.
Batteries are rated by mAh, which is basically how long they should last. I've seen NiMH AA batteries in 1200 mAh to 2700mAh, and it's definately worth paying for the better ones, and a decent charger (crap chargers ruin perfectly good batteries).
NiCds are usually 600mAh so they're pretty much useless these days.
I put my batteries into sets of four, and label them so I use and charge them together, (to prevent half charged batteries from flattening the charged ones, like RaveRepresent said).
If you want to be running something like a small stereo with speakers, you're probably best off getting a lead acid battery. Car batteries aren't suitable, as they're designed to put out a huge amount of power in a short time, and don't like being drained completely. The best bet is a sealed lead acid battery, and an inverter to turn 12V into mains power. Buying all the bits seperately can be expensive, so luckily there's a way to get them all in one unit.
You can get something like
this on eBay for about £50-£60 (half of that can be shipping as it's a bit on the heavy side). Xantrex is the brand to look out for, as they make some of the best 12V to 240V inverters, which is the most complex electronic part in the thing. Mine gets hooked up to a laptop and an old portable stereo, and I can do mixing pretty much anywhere. The thing has so much built in it's more use than a swiss army knife.