The extra 2 million pixels gives more cropping power which is important with a dslr on a scope and this helps bridge the gap with the higher magnification of spotting scope digiscoping. A dslr on an astro scope may only me useful up to around 4000mm equivalent but the excellent cropping ability of higher res sensors easily makes it a match against traditional digiscoping. Even better in my opinion.
Also DP Review said of the 450D "The new sensor is superb, and from a resolution point of view puts the EOS 40D to shame without losing any of the high ISO performance that has been Canon's trump card for so long."
Ideally you want to be going up in pixels which in turn allows you to use less magnification. Something like the 18 mega pixels of the 7D or new 550D would most likely only need the 600mm of the scope or at most the 1.4X TC most of the time.
For focusing you could try making something called a Hartmann Mask. It's basically a disc with 2 or 3 holes cut out of it. When the image is out of focus you see multiple images and when it is in focus you see only one. Take the disc off the scope to take the photo. I made one for when I take photos of planets like Mars, Saturn and Jupiter as it really helps get them dead on. I made mine out of an old lid that was roughly the right diameter to fit the scope. Even a thick piece of corrugated card would probably work as a test.
Some info here.
http://www.cloudynights.com/item.php?item_id=518
There's a template generator here.
http://www.billyard-ink.com/Hartmann.shtml
Paul.