Hi
I have a 24" iMac on my desk.
The desk is 1 inch from the wall and has a depth of 18". The iMac sits right up to the back edge of the desk, which leaves enough space so that the keyboard sits 3" from the base of the iMac and 3" from the front of the desk.
This looks good and is ok to work with although my hands do tend to rest on the edge of the desk when typing.
Just so you know the desk is also 48" wide which is enough space for me to have the iMac, keyboard, mouse and mouse pad, small Sony external speakers, a 6" bamboo one graphics pad, 2 ipod docks and a coaster for me coffee out all of the time.
I have a shelf in the foot well of the desk which holds the speaker sub woofer, usb HD for time machine backups, a 7 port powered usb hub and a second HD for backups on my MacBook.
I then have a separate set of draws next to the desk which has my all in one printer/scanner on top. It all looks neat and tidy, and fits in a relatively small space. The whole set up is 68" wide and 20" deep.
hope this helps you out, let us know how you get on.
Paul