Hi TK!
to the forum.
I don't
think anyone here has purchased the 400D yet, or if they have, they've been quiet about it so far
There are however, many 350D users and quite a few 20D(a) users.
I agree that noise may be a concern, but also the process of stacking and dark frames (not to mention noise reduction programs) will all help to reduce whatever noise may be present.
If noise is really an issue for you, especially on hot summer nights, you may consider getting a
cooled 350D, which Striker now has (do a search and you'll find his pictures of it, and using it). There's
significantly less noise in his test shots.
Is 8mp or 10mp going to make a huge difference for you? I don't know, I guess only you can answer that. For terrestrial stuff, a jump in 2mp wouldn't be enough to make me swap my 350D for a 400D, however if I was just buying a DSLR for the first time, I'd
love the opportunity to go straight to 10mp.
If you're using it mainly for astrophotography, have you considered the 20da? The live focus and better sensitivity in Ha might be enough reasons to go down that track instead of the 30D.
A few users here have the 5DH (modified for Ha sensitivity), with a full frame 12mp sensor.. the shots they're getting are incredible. Obviously a lot more expensive and may be out of your budget.
Anyway I hope i've helped a bit.