I cooled and monofied my 350D, see the
thread Here.
It worked great but the cooling was always annoying, if I did it again I would just do the mono mod.
I probably wouldn't going to all the trouble of cooling your 400D, I would spend the money on a newer camera that (uncooled) would almost be as good as a cooled 400D and save a lot of fuss.
If you were keen to go mono though, the 400D would be a good choice, but I don't think it worth the extra money to cool it. The extra sensitivity would somewhat negate the need for cooling anyway.
Cheers
Jo