Welcome to the world of modding.
First rule of modding you will learn is that anything can be done if you have a mind to do it.
Good wiring is probably one of the most overlooked, but very important, parts of putting together a good computer. This is because loose wiring will restrict good airflow. You can hide wires behind the motherboard, in empty drivebays and along the sides, bottom and top of your case by use of holddowns, ties, tape, etc. Just be careful to not put any of those thing on the motherboard. You can go to most computer stores like CompUSA or Microcenter or online at places like FrozenCPU, Performance-PCs, Voyeurmods, etc. and get different kinds of wiring management products. You can also go to places like Home Depot or any hardware chain and get doublesided sticky tape or taped velcro.
As for your PSU, the same thing can be done with it. Use hold downs, placed at specific spots in your case, to guide the cables and to hide the excess. I am assuming that you don't have a modular supply so you can just fold the calbes you don't use and use some tiedowns or holdowns to hide them.
Where are you located? In the US or abroad?