First off the odds of Nick beating GSP is almost slim to none. Having said that, should the unthinkable happen AND he felt retarded enough to challenge Silva then he would get an early retirement.
Silva is probably the worst possible opponent Nick could ever face. The only reason people were debating GSP vs AS was because of the X factor (GSP's Wrestling/Top control). Knowing that Nick would never take him down but stand and bang with him, Silva would literally kill Nick. Nevermind the size difference, Silva would put Nick into a career injury.