Right now there are only two GTD type apps worth looking at on OSX and they are Things and OmniFocus. Both have their unique way of doing GTD. My suggestion to you is to download the demos of both and see which fits your workflow and needs.
OmniFocus has a great white paper which really does wonders in putting the GTD philosophy into practice. Be sure to read it before doing anything:
http://www.omnigroup.com/ftp/pub/sof...dOmniFocus.pdf
I've tried both and in the end decided on OmniFocus because it suited my needs the most.