Most Realtors will tell you that the kitchen and the bathroom are the most important parts of the house.
Kitchen Counters
Countertops are highly visible, so if you're going to do any kind of rip-and-replace work, this is the place. Old Formica countertops should be replaced with at least Corian or Silestone. Ceramic tile countertops are debatable in terms of quality, but most potential homeowners are not looking for ceramic tile counters--replace them.
Kitchen Cabinets
Cabinetry is also highly visible because it is at eye level. Replacing kitchen cabinets can easily run you $25,000 or more. If your kitchen cabinets are in terrible condition or are made of melamine or
MDF, you may need to just bite the bullet and get new kitchen cabinets. For other type of cabinets, you may be able to re-face or repaint.
Quick Upgrade: Fixtures
Fixtures and hardware are surprisingly effective kitchen upgrades. Don't assume that replacing all of your kitchen hardware is going to be cheap, though. Replacing all of your hardware (i.e., getting rid of that fake gold plated stuff) can easily run in the hundreds of dollars.
Kitchen Sinks - Remove, Replace
Kitchen sinks are easy to remove and replace. Most DIYers can do this on their own.