First, don't get swayed by megapixels--packing more megapixels on the same size sensor is a real compromise, and unless you're always going to crop small portions of your image and then print large, you don't gain much with huge megapixel cameras. Plus going from 10-12, or 12-14, etc... is a fairly small percentage gain, and usually not material in image quality.
Second, definitely handle the cameras. I found the XSi body to be too small (and I don't have large hands). Also, larger sized bodies will have larger pentaprisms, making the viewfinder image brighter. (in fact, I believe that some of the smaller bodies, and the original Digital Rebel, have mirrors in the viewfinder image path, rather than a true prism--prisms are better)
I have the original Canon Digital Rebel, and recently bought the 40D. I'm a Cannon person right now. My brother is a Nikon person. Mostly preference, both are good brands. The consensus seems to be that Nikon, with their latest models, and especially their full sensor model, have pulled ahead of Cannon in image quality and features for the $--which is to say that for the longest time Cannon's were thought to have the slight edge in image quality because of their CMOS sensor (Nikon has started to use this type sensor I believe) and Digic processing...
Most people that debate the comparative quality of the two brands in the major photo forums don't seem to actually take a lot of photos, or if they do, it's for pixel peeping purposes to nit-pic. It's who's behind the camera that will have the most impact on photo quality. So don't get too swayed by any of the chatter on the forums.
As for lenses... I've got the original kit lens, 28-135 IS, 70-200 f4 L, 50 f1.8, and the Tamron 28-75, and 17-35.
If you go to the Tamron site, I think they've got a lens focal length comparison page, where you can interactively see the difference zoom ranges.
Try to figure out what type of pictures you'll be taking most of the time, and match zoom range for the lens. The ultra zoom lenses are getting fairly decent (like 18-125), but you still have image quality compromises at both ends of the zoom range. The upside is that you have only one lens to carrying around.
I go through phases where I really like what a wide angle zoom can do (my 17-35), but other times want to have more zoom power. The Tamron 28-75 is small enough and light enough to carry around as a second lens.
A consideration for the longer zooms (200 and above) is whether you can hold the camera steady enough to avoid motion blur... for me, my hands aren't as steady as when I was younger
, so an image stabilization lens (IS for Cannon, VR for Nikon) can be useful and give you 2-4 more stops of leeway in exposure.
Consensus (even by some diehard Nikon fans) seems to be that Cannon has a bit more selection in lenses, and more in the "L" class (i.e., $$$) than the comparative Nikon lenses. However, Sigma and Tamron have many highly reviewed lenses available, usually at lower cost than Nikon or Cannon--so you can't get too hung up on what's available directly from the manufacturer.
Good luck!!
