You'll almost certainly need to sand them and then varnish / stain. Before sanding you'll need to make sure all nails / screws are punched well below the surface of the wood, otherwise they'll rip the sanding sheets - your sanding company would probably do it for you - otherwise, it can be quite therapeutic going around bashing them into the floor (use a nail-punch and hammer).
If your house is old and has a suspended timber floor, you will probably find draughts coming up through the gaps in the floor. You can either fill these gaps (other more knowledgeable bods here will tell you how) or you can lift all floorboards and butt them closer together, inserting a new one in at the wall the width of any resulting gap. Or, you could insulate underneath them all with rigid insulation fixed to the joists, or quilt insulation suspended on chickenwire which has been nailed to the joists.
I was going to sand my circa 60 year old floorboards, but neighbours have complained that theirs are very draughty, and I didn't fancy the cost / hassle of insulating under them, so I just carpeted instead - much cosier!