Hi Chris,
to IIS.
To be honest you don't need ISO 3200 for astrophotography. 400 -> 1600 max is the norm. Above that and the gain produces too much noise from the sensor. I run my 450D at either 1600 or 800.
400D will work just fine as it is. The only downside to that model is it does not have Liveview which makes focussing so much easier with either Back Yard EOS or AstroPhotgraphy Tools so you can use your PC. The 450D is then lowest model with that very useful feature.
Nonetheless the 400D will still connect to a PC with BYE or APT and you can control the shutter and other functions. You can start by just using a remote cable and the inbuilt 30 second exposure setting for deep sky exposures.
I'd be saving the $$ for a decent mount because without that long exposures are not really an option. Video of planets or the moon is feasible. Does the 400D have a movie mode ?