i have a 1997 bmw z3 and it has a bmw "old fashioned" looking battery with the water holes, etc. I want to put a more higher performance optima or the like because i have stereo amps and other accessories. ive read on some bmw sites that you have to put a bmw battery in it because they are different, is that true? or are they just trying to get you to spend more money?

If you can find one the right size with the same terminal placement, sure, why not. but it's not going to help run a lot of accessories. For that you need more output from the alternator. That might be a little tougher.