I've been using iPhoto to manage my photos (successfully) for many years. I have multiple themed iPhoto libraries with many thousands of photos in them (ten years' worth of digital plus another fifty years scanned). I am thinking of downloading Aperture and giving it a try (thanks to an App Store gift card).
Can I use Aperture to edit photos within these iPhoto libraries? Do the libraries become "common" to both applications, or does duplication occur? I have limited hard drive space and don't want to create large duplicate files on my iMac.
It depends on what versions of iPhoto and Aperture you use, which in turn depend on what version of OS X you use.
Are you on 10.6.8 (that's what's listed with your post)? Are you planning on upgrading to 10.7.4?
If you upgrade all your software (which in turn also depends on whether you have the hardware to support it), then the difference between an "iPhoto Library" and an "Aperture Library" is null: the Library file format is identical, and every Library can be opened with either Program at any time. (Limitations are listed on this Apple support page.)
If you do not upgrade, then you cannot open iPhoto Libraries with Aperture. You must import them into Aperture, which creates a new Aperture Library, separate from your iPhoto Library. Aperture Libraries cannot be opened by older versions of iPhoto.