I have a Nikon D70 now and am considering D80 or D200 or maybe try Canon 30D. What should I do?

nikons lens quality is better. but it depends on what ur using it for. if its not professional then u can go with canon (quality of photo wont matter or its hard to tell difference if ur using it for everyday/trip photos)

Are you looking for better handling? Resolution? Low-light performance? Value? Each camera has its pros and cons.

The D200 is a great camera with excellent build quality. It is designed to be used by professionals, which means it's very durable and capable. Besides build quality, the biggest differences compared to your D70 are the additional 4 megapixels (66% increase in resolution, 10 MP vs 6 MP) and a much bigger and brighter viewfinder. If you like to manual focus your lenses, the viewfinder is a big improvement, and just nice to look through in any case.

The D80 has the same image quality (10 MP sensor) as the D200 and same large viewfinder. On the downside it isn't as solidly constructed, being plastic like the D70. This makes it lighter, which may actually be a benefit. Another tradeoff is the D80 lacks the mirror lockup function, which is important only if you shoot with a tripod. Finally the D80 can only shoot 3 frames per second while the D200 can shoot 5. These tradeoffs are ones that typically pros or very serious photographers care about. On the plus side, you save about $600 with the D80 vs the D200 ($1000 vs $1600), which can buy a very nice lens.

The Canon 30D is close in price and feature set to the D80, around $1100. The 30D has a little bit less resolution (8 vs 10 MP) but has better image quality at higher ISOs. This is very important if you like to shoot in low light, such as indoors without using a flash. The Canon 30D viewfinder is not as large and bright as comparable Nikons. Like the D80, it's not built as solidly as the D200.

If you have already invested in a number of Nikon lenses, it would make the most sense to stay with the Nikon system. Otherwise, you may want to handle both the Canon 30D and the Nikon D80 and decide which feels better to you.

I'm with Dr Sam. If you've invested in even one good lens, why move away from Nikon? This is admittedly subjective bias, but there's something better about the "feel" of a Nikon... go into a store and pick up a Canon... the weight and balance just don't feel the same. I'd recommend the D200 over the D80. The D80 is basically a D70 with more megapixels - nice, but is it worth the trade-in just for that?

I have a D70s and upgraded to the D200. It's awesome. If you have a couple of lenses, there's no reason to start over with Canon unless you are just not comfortable with your Nikon stuff.

Go to a local camera shop and feel them. The D80 is almost the same as your D70 and the D20 is quite a bit heavier. Some people like the heft and others are put off by it.

I have no experience with Canon SLR's, so I can't compare...

If you have liked the D70 then you shouldn't have too much of a learning curve with the D80 or D200. The D80 uses SD cards but the rest of your Nikon gear will fit. The Canon 30D is a good camera but I don't know that you will find it superior to the Nikons. If you haven't liked your D70 then go Canon and try something different.

Stick with Nikon. You would like Canon if you are a Nikon guy. D70 is a good camera. Upgrading to D200 would help you if you cannot get good pictures with D70.

First and foremost, cameras are only"clickers", people take photographs, whether you use a Leica, Instamatic, or anything in between.
In other words, if you are not Itzhak Perlman, a Stradivarius won't make you one.

You are heading down the long road of equipment addiction. Does your present camera actually hold you back in your creativity? Are you going Pro? Have you invested in any high quality glass or flash units? Whatever you buy today, tomorrow will present something better, newer, etc.
Ask yourself honestly if it's your equipment or you that needs some change. Have you truly outgrown your camera? Only you know.
(Go for the Canon 5D)