If you add a layer to your walls you may have problems with the look of your door & window moldings and baseboard. The trim problem won't look as bad if you use 1/4 inch rock, but the 1/4 stuff has its own problems (unless as Steve said the walls are perfect, in which case you wouldn't need to cover them).
If the door & window molding is very thick, as it often is in 1800s homes, you may be able to get away with 1/2 inch rock, but I would remove the baseboards and reinstall them later.
The right way to refinish the walls is to find someone to patch and skim-coat the plaster if it is not in too bad of a shape. There's nothing like the look of plaster in an old home.
The other right way to refinish them (particularly if you have to redo the floors and repaint the house anyway) is to knock out all the plaster, leaving the lath in place, and then cover the lath with 1/2 inch or better rock. It is a dirty and dusty job but it goes surprisingly fast. Hire some kids, give them dust masks, hammers, shovels, and their favorite music, and get a construction dumpster for the waste.