mac2sell.net is a good online resource, but they give a bit of a lowball figure. I'd check craigslist in your area and see what they're going for.
They're still popular machines, and still quite useful if the ram is maxed out like yours. If you list it, make sure you mention that the ram has been upgraded, and list any extras that you'd be throwing in, like a case, extra battery, etc., or any software that you're willing to leave on it, like leopard, a recent version of iLife, iWork, Office, Creative Suite, etc.
Honestly, why wouldn't you be able to use your powerbook for law school? I've got your machine (well, not literally) and it is perfectly good for word processing, email and browsing. Your PB is a good size and build quality to throw in a backpack and take to class. Assuming that lexis/nexis and westlaw will work on your browser (I'd recommend safari 4 if you haven't done so already.), you may as well leverage every bit of life out of the system you've already paid before before incurring even more debt.