You have a camera with a "state of the art" sensor. The size and quality of your enlargements is only limited by your abilities and the quality of the lenses attached to your camera. My son has a D5000, and I had a chance to use it recently. I think that it is a very nice piece of equipment.
If it's within your budget, you may want to consider buying a printer that can handle large prints so that you can do your own printing. I have a Canon Pixma Pro 9000 that handles prints up to 13 x 19, and Epson also makes some nice, reasonably priced, printers. Printing your own pictures is part of the fun, and it allows you to control important aspects of the final product.