Check the zooms on those cameras you listed as I suspect that the figures quoted are multiplications of the digital and optizal zoom (most cameras have both) It is rare to see a camera with optical zooms greater than 10X - 20X, so you might need to look into an adapter to get the reach you need (you will need to make sure the camera you get has the option to add accesory lenses or has a standard photographic filter adapter as many of them either have a propriatory fitting or a non standard thread size)

size and weight are normaly listed in the product specs, but a trip to your local good camera store definately helps as they should be able to let you hold a couple of models so you can work out what suits you best.

As for lugging a heavy tripod, if you spend the money you can get carbon fibre tripods, but for anything over about 10X zoom the image stabiliser in the camera is pretty useless, so your footage will be as good as the stability of the camera. In the field you might find photo clamps onto trees etc and solid mini tripods on a rock are as usefull as a big tripod, but that really depends on where you are going and there really isn't a subsitute for a solid tripod, especially if you will need to pan or zoom the camera whilst recording. if you are going to set it and leave it you will be able to use a much lighter tripod.

In terms of mac compatibility I am assuming you will use Imovie to edit ? you really can't go wrong here. With linux it really depends on the distros and how they handle peripherals, is the camera going to use firewire and does your linux distro support that? or is the camera usb mass storage class or some other format? I have been meaning to play with ubuntu studio but I don't have a spare computer for it at the moment so I haven't really explored that option.

Sound, good sound will make a big difference in quality, but in your budget you don't have a lot of options. most reputable camera manufacturers have pretty good built in mics these days, sometimes even 5.1! but they will still pick up your noise and any button pushing etc you might make. a mic jack on the camera and or a propriatary hotshoe zoom mic will help but you then have the expensive of buying that extra mic. trial and error is the only way out here, get the camera, see if the sound is ok if not get the extra mic.

Low light - some manufacturers offer stupid low light settings 0 lux type of stuff. the problem is there are two ways to acheive good low light and they both have issues. the first is using infra red spectrum recording wich gives that characteristic green shot. The second is to electronically boost the recorded light levels (boosting the gain) this results in a washed out fuzzy image known as gain grain. It looks terrible. again if your local store lets you try recording some dark areas of the shop or if the shop is brightly lit the inside of a box or bag with something to focus on inside it. also when looking at the camera specs err on the side of cameras whose lenses naturally let in a lot of light (lower F number for the aperture/iris value)

also check out the links in my signature as they talk about some of the background technologies that will help you make sense of the many many formats that are now available and this should help you decide what format is best for your needs and keep your budget down.

I hope that has answered everything. if not ask more! and when you get your camera write us a review and post some footage if you can!