I recently came across an "old" iMAC "teardrop" style computer that I plan to use primarily as a cruncher. (A little POGO spades as well as a sop to the wife) An aquaintance is going to do a re-install for me with OS 9.2 I believe. Are there any projects that favor this OS as opposed to Win XP? Also, any links that could be provided to a noob like me for a first time MAC user would be greatly appreciated.

Pump it up to maximum RAM, put OS X 10.3.9 on it and run BOINC. The projects that have PPC applications are fewer and fewer these days, but that's probably your best bet for good grid computing. A light version of Linux (maybe one of the BOINC distros) might give you a little better performance, but I'm not sure how many PPC/Linux apps there are out there.