Wikipedia's article on AGP includes a short section on Compatibility that may be helpful in sorting out the odds and probabilities while you're swapping boards aroung.
Within, or immediately adjacent to the compatibility blurb there should be some pictures of the voltage slot/pins that will let you read off what voltages your sockets provide and what voltages your various cards are intended to use.
A not at the beginning of the Wiki article indicates that whoever wrote it didn't provide citations for many of the "assertions of fact," but it appears to be an informed essay(?).