iPod Batteries.

Evan noticed recently that my iPod sounds kind of crappy. It isn't noticeable when you're just listening to it with the earbuds, but when it is hooked into our stereo system, it can't quite handle the base output too well. My brother in law thinks that possibly the battery is going. I've only had the thing for a year, but I have no idea how long the battery life is. Anyone? Anyone? Also, are there different 'levels' as far as quality that I should be considering when I buy a new one? Are some better (read: longer-lasting) than others? Can you use rechargeable batteries? And, aside from Apple, does anyone know the best place to buy them? All recommendations and suggestions for finding excellent ipod batteries wanted...!