More PC's means that the party as a whole gets more actions per round and therefore can bring more pain down upon your bad guys. Particularly if the adventure has encounters with a single BBEG or large monster like a dragon. It also means that the party has more resources (read: spells, hp) per day. However each individual character is no tougher or more capable than normal so you must be wary about raising numbers they have to roll against (DC's, AC, SR, DR) or else they will be unable to cope or worse wiped out completly. With that in mind...
Things not to do: give more than 1 or 2 more levels to your bad guys, replace monsters with tougher versions of themselves, add more bad guys of an equal level to the current bad guys
Things to do: add lots of mooks as bodyguards/cannon fodder around the BBEG's (bonus: this gives the cleave fighter, undead turners, and fierball slingers a chance to shine), also maybe add an extra encounter or two with a below-average EL in order to make the party use up some of its extra resources.
Above all try to make sure that everyone gets a chance to participate and not be left out (this goes along with the theme of making encounters bigger rather than tougher).
hope that helps.