Basically there are three lens price ranges-
There is the kit lens price range- and Westside guy is right- the 55-200, and the 18-55 are probably your best bet. You can go and spend some money on something like the 28-80 f/3.3-5.6, 28-105 f/3.5-4.5, or the 24-85, which will not be that great of upgrades, and basically you'll be spending money without much improvements.
There is the midrange lens prices, which range from $400 to $1000, and include the all-purpose 18-200, 24-120, and the 80-200 f/2.8. That's clearly out of your price range, but the glass you get is head and shoulders above the rest.
If you want the best out of your camera, and have money to burn, you can go for either of the lenses I suggested in my first post, or the 70-200 VR. Mind you these cost $1000+.
Your only other cheaper option to get better than the lenses you have, is to go prime. Prime lenses are much sharper and faster than any of the other lenses (even the high-end ones), but you will lose the flexibility of a zoom.
So, yeah- welcome to SLR photography. Glass is the most expensive thing here. Good news is that if you decide to wait, save your pennies for a while, and get some good stuff, you'll have it long after that D50 bites the dust.
