http://www.m3post.com/forums/showthread.php?t=400599
I gave my 2 cents on this topic a while back in another thread posted above
You don't need high pressure, you need water volume - gpm (gallons per minute)
Modulating pressure through tip selection utilizing interchangeable quick release tips with varying pressure ratings can turn any power-washer in to a much more versatile tool. Water volume remains constant (this is the job of the pump) while the pressure is regulated (up or down depending on the diameter of the opening of the tip) by the restriction that the tip provides up to the max pressure point of the pump. Most pumps are rated beyond the rated pressure for the machine and can be run above that, although it adds wear and tear to the pump.
There is a simple calculation used for "cleaning units" which is a loose measurement of cleaning power of a set up.
pressure x gpm = cleaning units
Example.
3000 x 3 gpm = 9000 cleaning units
2000 x 5 gpm = 10000 cleaning units
So - the 2000 psi 5 gpm machine is actually more powerful than the 3000 psi unit
You also don't want to use a power-washer to apply soap or foam. You end up using so much water (because the gpm is so high at 2-3-4 gpm) that the soap is so diluted that it's ineffective, or you'll use way too much soap and 80% will end up on the ground. Buy a standard garden sprayer and premix the soap in it then apply with a fan or cone tip.
I would suggest a 2500 psi machine at 3-4 gpm and utilize larger tips that bring the pressure down to 1500. If you buy a machine that's only 1500 psi the functional gpm will be too low for varied use.