Most rear deraileurs sold in bike shops today start out at 8 speeds, so chances are your deraileur will accomodate the shift just fine (having a deraileur with a higher gearing range than you need is generally OK; as long as you don't have a 6 speed deraileur, in this case, you will be fine). Your shifters, however, may still be 6 speed and may or may not accomodate that 7th shift in indexing mode (as others have pointed out). If you have any urge at all to upgrade shifters at all (i.e. from downtube to bar-end, or from bar-end to brake lever shifters), now might be a good time to do that, since you are upgrading your gears, too. Generally, Shimano shifters will be compatible with Shimano freewheels, except when it comes to Dura-Ace (either all Dura Ace or none at all is the general rule). I would also try to get shifters that exactly match your gearing in the back (I just got 8 speed barend shifters for my 6 speed bicycle, and I'm having a heckuva time getting that indexing to work).