>I'm looking for advice on a good piece of glass to invest in >for my D200. I currently have a Sigma 18-135mm, which was >fairly inexpensive. > >I've been told to invest in a good lens...it can make a world >of difference. > >Any recommendations? I'd like to stick to under $500, with >the same focal length, possibly a bit wider (10mm or so).
Well, you don't say what your subjects are, so it's a bit hard to give a good target lens. You also cap your budget at a price that takes nearly all decent glass off the table unless you are buying used. And even then it would be difficult.
So I'll try to offer the following advice.
Nikon's G series prime lenses are probably going to work beautifully on the D200. Any of the AF-S G series zooms are probably going to work well also. Any zoom with goes beyond a 4:1 ratio is probably not going to be much of an upgrade.
Safe bets are any zoom with a F2.8 max aperture, or primes with an F1.8 or larger max aperture. While there are a few exceptions, staying within those guidelines should put you on the path to "good glass".