I swapped out my MTB tires on my Gary Fisher hardtail for Specialized Nimbus tires (26 x 1.5"). The tubes ran $8 for both, and $32 each for the tires at my LBS. I'm sure you can find a better deal online, but even at that price it was totally worth it. I've jumped curbs, ran through a patch of goatheads, over glass, etc. and been very surprised to rarely get a flat. I went from pedaling at about 14mph on my MTB tires to 17mph with the road tires. It was a great upgrade, and a very noticable change.
http://www.roadbikereview.com/cat/wh...8_2489crx.aspx