Removing artificial invitation limits is key if you really want to go viral. Most places have people called "whales." They are the ones that send out to their entire address book, while most just send none or one.
However a cap does serve its purpose for scalability and hype reasons. To which you choose, is probably a function of how engaging the app is outright.
Virility is something else, and is largely unaffected by startup metrics. Look it up: http://dictionary.reference.com/browse/virility
Finally, check out my blog post about how to make an application viral: http://danieltenner.com/posts/0009-how-to-make-your-applicat...