I've not used a D50 personally, but from reading up on it in the latest Outdoor Photographer magazine (they've got a mini-review of it), it seems like it'd be worth spending the extra and getting a D70 or D70s.
The 350D I've not used either, but from what I hear it's a very nice camera. I played around with a 20D a couple of weeks ago with a 100mm f/2.8 USM Macro lens... VERY slow to autofocus, but incredibly sharp images.
Personally, I prefer Nikon, so I'd lean towards the D50 or D70/D70s (or even a D100). It all really depends on WHY you want to go with an SLR - and how much cash you want to spend.
Are you just wanting to take good quality photographs? Or do you REALLY want to learn photography and upgrade to professional gear in the future?
If you just want to take good quality photos, and keep the camera for a few years, go for a D70 or D70s (or D50 if your budget doesn't stretch to a D70/D70s). If you want to eventually progress to professional level equipment, go with a D100 and the MB-D100 vertical grip. The D100 has a magnesium alloy body (the D50/D70/D70s have a plastic body), and it's a much more substantial feel to it, but at about the same price as a D70 now - again, I shoot Nikon so I can only speak for their cameras.
As far as lenses, with SLR, all there is is "optical". Whether you get zoom or not depends entirely on what lenses you buy.
Personally, I like primes (fixed focal length lenses). I have the 50mm f/1.8D and the 300mm f/4D ED-IF AF-S. I intend to add an 85mm f/1.8D and the 105mm f/2.8D Micro-Nikkor. You generally get a MUCH better quality of image with primes over zooms.
With the D50, D70 or D100, you'll want to get the 18-70DX and the 70-300mm f/4.5-5.6G lens. Those will give you a good range to get you started. The 18-70DX is around $250, the 70-300G is around $125.