Colin,
In terms of positioning the TV, there have been a lot of discussion about this on the forum. Most people opt to position the screen so your eye level is somewhere between the bottom and the middle of the screen. Again, this is down to personal preference, but if the screen is too high you may find you'll get neck ache looking up at it!
Check out Dolby's website for advice on speaker positioning:
Dolby - Dolby Room Layout and Speaker Setup Guide - Speaker Placement for Home Theater Surround Sound
They recommend that the front three speakers are positioned vertically so they are level with your ears. Most people tend to place the centre speaker below the screen, but again this is personal preference - you can place it above or below, just try to get it as close to ear level as you can.
When it comes to positioning the speakers horizontally, there is advice on Dolby's website when it comes to the angle between the front two speakers and your listening position - the same goes for the rear speakers. I followed theory for my setup, but you may want to group them either side of the TV so it is more aethetically appealing.
Cheers,
Matt