I think the Shimano 600 derailleurs maxed out at 7 gears (this for the 600 Ultegra, pre-1998 according to Sheldon Brown--scroll down to "Older Group Names"). You'll need to replace the derailleur to one compatible with an 8-speed drive train; I'd suggest geting an 8-9 speed derailleur, just in case you decide to add another gear in the future. (You do need to make sure the new derailleur is also compatible with your shifters.)