The 40D is a more near match to the D300 but even then I would probably give the edge to the Nikon.
Even then they're not really comparable, the D300 costs about 80% more (£500 vs £900 body only). Although, I wouldn't mind one

The 40D is a more near match to the D300 but even then I would probably give the edge to the Nikon.

Is that really a serious question? The 40D will be lighter and will have a better LCD, but I can't think of another advantage that it could have.
Yes the 40D has got slightly more MPs, but in the real world, there's probably no significant difference in absolute resolution.
With the 1D you'll get all the lovely bits of awesome AF, faster FPS, better metering, effectively a built in grip to allow vertical use seemlessly, weather sealing etc.
The key thing I saw from moving to a 1 series was a serious increase in the "keeper" rate from shots. So stuff that my old camera would have left unfocused, the 1 will do a much better job of. You can have as many MPs as you like, but if it ain't in focus and metered well, all a bit pointless really.
I really would only consider switching if you feel you can handle a Nikon easier on the fly than the Canon.
johnny
- I was merely saying how impressed I was with it 

Yes the build quality is better, but again the 40D is massively improved over the 10/20/30/5D in that respect too.
No It hasn't. It's largely the same asides from a few ergonomic differences and the very limited sealing. I've used a 20, 30 and 40D and they're all largely the same regarding build quality.
(i still own a 20D and had a 30D before the 40D, and regularily handle the 5D and 1DSmk3)
The 10/20/30/5 rubber all feels very plasticy, much nicer on the hand the 40DAlso, (again comparing it to the 1D which is where my statement was made not the mk2), the high iso noise is absolutely dreadful compared to the 40D, the colours arent as good, there is a sizeable difference between an old 4mp CCD and a modern 10mp CMOS.........Yes the build quality is better, but again the 40D is massively improved over the 10/20/30/5D in that respect too.
You're right that the high ISO capabilities are clearly better on the 40D, but I'm surprised that you feel that the colours would be better, bearing in mind that much of their accuracy is down to the metering, in which case you'll be comparing a pro-sumer set of metering to a pro.
As for the detail, really depends on what you intend to do with it. If you have a requirement to do lots of cropping or need to print seriously big, then 10mp is a good thing. I'd still argue that actually capturing the shot is a lot more important than out of focus detail that is only viewable when you pixel peep.
I guess it really comes down to personal preferences. I guess you prefer to not have to carry around another kg in weight.

I had a play with the D300... and bought one!
Paired with good glass its a great piece of kit and reassuringly well built.
You just need to grow some more musles Jonny.Seriously though, with the grip it does beome a big hevy block of metal. You do get used to it though.