Pressure washing is not always about the amount of pressure you use. It's more about the Gallons Per Minute (GPM) and the proper chemicals/detergents. 3000 psi is more than sufficient for most jobs with detergents. Spraying brick with Oxalic Acid works wonders. Many times we don't even need a pressure washer to clean a lot of our properties. Hypochlorite is the active ingredient in bleach and chlorine which works wonders as well depending on the type of stain. You can get chlorine from a pool supply store in 2 gallon containers for cheap (about $1/Gal). Anything black or green can usually be taken off with chlorine unless it's oil based. Anything red or real difficult to remove can be taken off with Oxalic including many oils. You have to let both of these chems dwell for at least 10 minutes before cleaning off. Use lots of water! http://www.thegrimescene.com
is a great forum to learn - it's for contractors but if you need specifics, you'll find them there for sure.
Good luck guys!