Yeah-its a real temptation a 5D2,that Humayan and his 'Venus Goddess' on the beach
(stunning image) I imagine has got many thinking 5D2-I am pondering the very same thing,Mike.
The cheapest I could find a new 5D2 for was around $2200 at DWI,but that was before the Earthquake over in Japan,Unfortunately the Canon gear has gone up a bit since then,I looked at a 100 mm EF macro (non L) in November from them it was prices at$517,now approx $570.
As for selling your two 40D's,depends on age etc etc.But I think a really good example of 40D might make $700 plus,a mod'ed one really not sure,but at least the same-plus a bit.So may be you would get $1500 towards the goddess excursions.
The low noise on this 5D2 sounds very good.One thing I was a little suprised about the 5D2 was it was only 3 FPS (I may stand corrected on this),which would be a small issue,as I would want to use the 400 mm L on it for birding.
Good luck,with your decision,I feel,that once I have done the astro imaging I want to do,I will sell one of my scope set ups,and put funds to a 5D2,I am sure it would get a lot more use.
Chris