Finishing your basement can be by far the least expensive way of adding quality space to your home in location. But be prepared to confront the structural challenges. To make the space comfortable and attractive, you have to first deal with the pipes and ducts and other denizens of the deep that are there for good.
Start by evaluating moisture problems. If there's dampness, you have to get rid of it before you can do anything else. The second big challenge is figuring out how to heat and air condition the space because it's very likely that your HVAC system isn't powerful enough to heat or cool such a large area.
If you are contemplating putting in a bathroom - a good idea since otherwise anyone who spends any time down there will have to run up and down the stairs - you'll want your plan to include leaving the plumbing where it exists. While cutting into a waste pipe or a supply line isn't a big deal, it can be difficult to relocate them. So plan the rest of the layout around the fixed location of the bathroom.
After that you'll want to think about ceilings. While a standard drywall ceiling is attractive and gives you more headroom, it can be an impractical solution for a basement if you need to get to pipes and ductwork above it. Many people find that a suspended ceiling - while not as esthetically pleasing - works better.
Attractive flooring is one of the most important factors in making a basement comfortable. Anything that you'd put upstairs works just as well downstairs except hardwood flooring. It's not a good idea because if there is any moisture the floor will be ruined. The floor will be warmer if you add an insulating sub-floor instead of installing floor covering right over the cement slab, but that will steal headroom.