Join Date: Jun 2004
Location: Northern Nevada
Tubes are interchangeable between 700 and 27, but not the tires. Most bikes sold in the U.S. until about the time yours was made came with 27-inch rims, while most European bikes used the slightly smaller 700s. By the '90s, I think (I forget exactly when) 700 had pretty much become the road standard. If your tires are marked 27-inch, that's what you'll need to buy.
There's no particular advantage to either size, though as 27 fades, the range of available tires has shrunk. If you ever have to replace wheels, you might check the brakes to see if the pads can be moved enough to allow you to use 700C rims--it's a minor adjustment if they can, and you'd greatly expand your range of tire possibilities. Probably not worth spending the money unless you need new wheels anyway, though.