I have used a couple of modded Canon 20Ds and a Nikon D70 modded.
They are all fairly easy to mod. I am currently modifying a Sony Nex 6 which is more of a challenge as it is so small.
Modded DSLRs are way more sensitive than std cameras. Std cameras have a very strong UV/IR block that cuts out Ha. Removing it gives about 3X increase in sensitivity roughly. But it does wreck it for daytime imaging although you can put an Xnite CC1 filter in front (that's one option).
Processing simply means you correct the red bias in the image using levels and curves in Photoshop. I have quite a few modded 20D shots I can link here:
http://www.pbase.com/gregbradley/can...0d_astroimages
But of course its up to you and what you want the camera for.
Greg.