Nikon D80 or Canon 30D?

[tomcomleyphotography]

Hi everyone,
A little while ago I posted a similar topic comparing the D50 and 400D, but never purchased either. I am going on a holiday late this year and possibly late next year also, and want a new camera for this. I currently have a Panasonic Lumix FZ30 and feel it is somewhat limiting with a fixed lens (which has a minimum focal length of 35mm). I am currently tossing up between a new DSLR or new computer, but if I choose to go along the lines of a DSLR, I will need some advice. I have a few questions and comparisons to make.

First of all, why do lens' vary so much in price? Even same brand lens's with similar focal length seem to range a lot. I know about VR and IS but is there more then this? Thousands of dollars more?

Secondly, I was considering the Nikon D80 and the 18-200mm VR lens, which costs about $2000 on ebay. I was considering my options when I took a look in the Canon section, and found a 30D with 18-55mm lens for near on $1,200, then the 70-300mm IS lens for roughly $800.
So they cost roughly the same, but the Canon has an extra 100mm zoom and the CMOS dust cleaning. I've heard the D80 is very good but haven't read much about the 30D, so I was wondering if anyone here with first hand experiences with either camera or either lens could leave me a response of any kind, I'd greatly appreciate any help.

VR or IS is important to me also, as I'll be photographing motorsport, normal sport and extreme sport, as well as being creative around the place and taking some landscape photos. I was also thinking about a macro lens but that might bump the price up a little.

Oh and do't take notice of my album on here, the photos were taken with a handy cam, so here's my photobucket album:

http://s54.photobucket.com/albums/g105/ ... otography/

And if anyone needs, here is some ebay links:

Nikon D80 (with 18-200 VR)

http://cgi.ebay.com.au/Canon-EOS-30D-AU-18-55mm-Lens-kit-2GB-CF-6Gifts-AAOM_W0QQitemZ230179906984QQihZ013QQcategoryZ113382QQtcZphotoQQcmdZViewItemCanon%2030D%20(with%2018-55mm)

[url=http://cgi.ebay.com.au/Canon-EF-70-300mm-f-4-5-6-IS-USM-Lens-5D-30D-400D-0391_W0QQitemZ230180522235QQihZ013QQcategoryZ30066QQtcZphotoQQcmdZViewItemCanon 70-300mm IS

Thanks for looking and thanks for any advice!

Tom.