That's a great price, I recently paid the equivalent of $130 for a 320gb, 7200rpm Hitachi hard drive for my Macbook, so $115 is brilliant for 500gb. Just a few things to bear in mind; warranty should be 3 years for WD hard drives IIRC, so if you're picky Seagate have 5 year warranties on theirs (just in case you find a comparably priced seagate). Another is the speed, 5400rpm is what you're used to, but there is a noticeable speed difference if you buy a 7200rpm drive when transferring large files (or re-installing leopard). For me, the worst thing I've noticed from my new hard drive is more noise, although it can only be heard in a silent room, it does annoy me slightly (not sure how the WD will fare, should be just as quiet as your current hard drive though). Finally, I'm not sure if you have the new alu-Macbook or old, but if its the older version, you'll need a Torx T8 screwdriver to pop the hard drive out from its special case-thing.
Sorry for the essay, hope it helps.