Full costume is a must!
I wore the kilt and puffy shirt a few weeks ago... go for it. It adds to the fun.
You usually get free entrance for full costume dress
If you do, try to get something that is really from the Renaissance era. Too many people at those things are dressed like it was still the Dark Ages.
I've only been to one but I go the impression that a lot of the people weren't really very well informed on the Renaissance period. But the, I have a history degree so I shouldn't expect everyone to know I guess.