Assuming there aren't any structural or critical repairs you need to do (fix the roof, replace the furnace, update the electrical), then I'd be tempted to sell "as is" for a lower price an not worry too much about it, just fix obvious things like peeling paint, broken door handles, things like that. Give the place a good cleaning (better yet, hire a cleaning service!) and declutter if you're still going to be living there while it's on the market.
But, if you'd rather take the time and see if you can boost your selling price, then the two places I'd sink my money are the kitchen and the bathroom(s). These are the areas where you will most likely get back more money than you invested into to updating. Paint, replace counters/vanities/sinks/toliets and if you have the money and it's needed, the kitchen appliances. If the appliances work okay but are just an ugly color (burnt orange, olive green, etc.) you can try painting them with appliance paint to give them a new look. Also, if the cabinets are out of date, see if you can paint them. If they are wood, just sand them down, prime, and paint (white is usually your best bet) and maybe update the hardware (handles etc.). Counter tops are the worst, they are expensive to replace but if they are shot there isn't much you can do usually to save them, you may have to have them replaced no matter what. Same with the floors. And of course, fresh paint on the walls is always a safe bet. If they have wallpaper, try to remove that rather than paint over it. Sometimes removing wallpaper can damage the wall, though, so be carefull. Go with more neutral colors, it makes the rooms look bigger and people have an easier time imagining their 'stuff' in the rooms.
For the rest of the house, if you have carpet, but you know there are hardwood floors underneth, see about getting them refinished, hardwood floors are a really hot item right now, and refinishing is almost always cheaper than getting new carpet. And again, take down the wallpaper (if any) and paint!
If you have cable tv, watch "Sell This House" on A&E, and I think there are a few 'decorating on a budget' shows on HGTV too. Tape them, watch them, love them, use them. They have awesome ideas and examples of cheap/quick fixes that go a long way!
Good luck!