Cu_amiga:
1. No sadly not, but most are!
2. if you are going for mediator, go for P8/P9 solution psu and not ATX , if you go for mediator SX then go for ATX psu (SX have AT and ATX , Mediator (normal) has only AT.
3. Any pc store, just a normal one, 200 watt and up!
4. sorry dont got a powertower so i dont know, but i cant think of anything really, but i am sure powertower use AT styled psu, or?
goodluck!