I am probably biased but I would recommend Folding@Home. Whatever you choose make sure you get in with a team that is active. I did a BOINC project a few years back that I dropped within a week because there was no active community.
Come check us out and say hello here
As for stability, F@H will consume every resource you allot it, but you can scale it back if you are uncomfortable with the stress on your system. The client installs are sometimes confusing but we are always willing to help with any questions.