Wayne ... you have several choices, based on quality and price. The most affordable, I think, is a third-party lens, which I use -- the Tamron 28-75mm f/2.8. It's a very good quality lens (I truly like it) and a constant fast aperture (2.8). It retails for $400 at Amazon.com.
Because your camera body has a crop sensor, you can get a Canon-made EF-S 17-55mm f/2.8 IS. This is very HIGHLY regarded lens -- considered a virtual L series lens except for two things: it's made of plastic instead of magnesium alloy and it is not weather-sealed like the L series lenses. Otherwise, it's premium glass. Because of the very high quality nature of the glass, it's pricey -- just over $1,000. But you have that beautiful constant f/2.8 aperture AND the image stabilization. The one drawback: If you upgrade your camera body, at some point down the road, one one with a full-frame sensor, you won't be able to use this lens on it.
If you want ultimate quality glass, then maybe you should consider the 17-40mm f/4.0L. The aperture isn't quite as fast as the 17-55mm and you don't have image stabilization. Price is $700, which many would say is a great price for an L series lens, and it probably is. If I were you, and I didn't already have my Tamron, and if you could afford it, I'd go for the EF-S 17-55.
Go for a 24-105mm f4 L IS lens. Best in it's class and available on ebay for a lot less than list price. I paid £500 for mine and never regretted it, especially when I upgraded my camera from 400d to 5D MkII.
Have you looked at the lens Canon EF 28-135mm f/3.5-5.6 IS USM Standard Zoom Lens for Canon SLR for $407.99 at www.amazon.com? I have not shot with one but I have picked it up and felt the lens, also on a camera where one was mounted on a 50D in a Bestbuy store. It seems to be a good lens for the theme it was designed for. Just a thought worth mentioning since you the question out generally.
I also have the 24-105 mm L lens. Nice replacement. I never used my kit lens that I got with the first DSLR camera I bought which was the 20 D. I sold it right away.
I also sold my 28-135 mm that I got with my 40D. It was not as sharp as the L lenses.
24-105 mm L IS USM f.4 is a good lens and very versatile. But of course it all depends on what kind of photography that you do most of the time and how much money you would like to spend. I use my 24-105 a lot both for portraits, interiors and landscapes. I have other lenses that does some of these jobs better in certain circumstances but in general i think this lens can do a lot and give a good result.
Is your lens the IS model? If not, an inexpensive upgrade would be the 18-55mm IS lens.
Otherwise, the 17-55mm f/2.8 IS lens is great and is what I use in conjunction with a 70-200mm f/4L IS on a pair of 1.6x cameras. However, the 17-55mm f/2.8 IS lens is an expensive piece of glass.
At a lower price with no really great reduction in quality but, with a lack of image stabilization (which is not really a great deal in a lens of this focal length) is the 17-50mm f/2.8 Tamron.
With the exception of the 18-55mm IS lens (which I list because of its low price) I would normally stay away from:
1, Extreme focal length range lenses since these lenses most often have lower IQ, slower auto-focus and always slower f/stops.
2. Any lens that is slower than a constant f/2.8 aperture for my mid-range zoom selection. IMO, f/2.8 is really necessary in your most used lens.
Sorry for taking a while to reply and thank you for all your comments, i have decided to upgrade my camrea as well so i am going for the D50 with the 17 - 85mm is lense. I have heard that is a good lense.
The 17-55mm f/2.8 IS lens is an awesome lens and I dearly love mine. I was shooting some bounce flash stuff at a Christmas party. It was off and on shooting and I neglected to turn on my flash for a couple of shots. I was shooting in the programmed mode at ISO 200 and the lens produced some very sharp images; one at 1/15 second and the other at 1/8 second. However with the awesome imagery, comes an awesome price. If you can afford the outlay, the lens is magnificent.
As an alternate, I would select one of the two Tamron 17-50mm f/2.8 lenses. I would prefer the lens with Tamron's version of image stabilization but, could make do quite well in this focal range with the non-stabilized model.
BTW: I think you meant a 50D camera. The D50 is a Nikon...