Yeah, it may have...although it may have made the whole impeachment more legitimate in the eyes of the voters too. A large majority opposed impeachment, but if Clinton had resigned, it would have made the entire thing seem more legitimate. Hard to say exactly what the impact would have been...being the incumbent may have been an advantage for Gore, but maybe not, depending on how he performed.
Gore would have won without the Lewinsky scandal, so if clinton cut it short by resigning, he would have helped out Gore.
