I would guess that razorback is thinking about exhaust cleaning as that is the forum he posted in...
You will get many different answers anywhere from 3-5 gpm and 2000 - 4000psi but the one thing that you cannot do without, I think everyone will agree, is HOT water. Everyone will have different opinions on brands etc. We use Landa, yes they cost more up front, but I know we can depend on them at 3 o'clock in the morning!