I'm sure this is a dumb question, but I don't want my cables to end up short, but also don't want to pay for more cable than I need.

The run will cover ~21 feet of ceiling space (via the attic), cross roughly 8 feet, then down about 7 feet to the equipment rack.

Is 40 feet enough?

Also, can you get 22 AWG HDMI cable thru the cable wal plates on Monoprice?

Are you connecting source to source and using the Recessed Low Voltage Cable Wall Plate? If so I would get the 45' cable I hate when you think it will be long enough and then when you want to move stuff around its just 6" short :( Never hurts to have a little extra cable. One thing about the 22awg cable it is pretty stiff

The wall plates on mono price look like the ends will fit with no problems
http://www.monoprice.com/products/product.asp?c_id=105&cp_id=10425&cs_id=1042505&p_id=3997&seq=1&format=1#largeimage