I have the Otter Box and LOVE it! It offers several layers of protection. This site has it for the cheapest I have seen!
I have the defender case also and love it, but I think that may be overkill in this particular case. A better solution may be the OtterBox Commuter case w/ Screen Protector. Not nearly as bulky as the Defender. I also think the CaseMate Barely There is an excellent choice for this scenario.
I have the Blackberry official 8520/8530 skin, with is great for scratches and dropping from short distances... I also have WriteRight screen protectors...
Might be just the ones I have but the ribs are not interchangeable to the base that it didn't come with, ie the bright pink one didn't fit on the black base. I got them for $20 including shipping on ebay.