Room is 20 ft wide x 17 ft deep...

PJ image will be on the 20ft wall with front channel speakers on either side.

Q: What would be a good separation distance for the front speakers? I'm thinking 8-9 feet...

You can work it out from the Dolby Labs recommendations--

http://www.dolby.com/images/consumer/home_entertainment/room_d4.gif

For music the usual suggestion is an equilateral triangle.

For home theater bear in mind that if the mains are far away from the display or screen sounds will come from off camera when the action is at the far left or right or will seem to be way to far off center; the knock on the apartment door that sounds 40' away.