What is the minimum PSI required for a pressure washer to remove spray
paint from a sidewalk?  Is there a strong enough electric pressure
washer that can remove spray paint, or only gas powered is strong
enough??

Removing paint from concrete can be a real problem. Just picture the
last time you had to remove dried paint from your hands after a
painting project.
Concrete, just like your hands, has pores into which the paint will
adhere. Remember when washing your hands you had to do much more than
just turn the pressure up on the tap.
A better result is obtained with hot water. If this solution is not
available chemicals designed to break the bond between the paint and
concrete will have to be employed, a long handled stiff bristled brush
will be required to aggressively work the chemical into the surface.
Plan to give the chemical solution time to dwell you may have to turn
on the sprinklers or mist the surface to allow the chemical to dwell
without drying.
With the proper chemicals psi ranges of 2000- 3000 can be effective.
To answer your question more specifically straight water pressure is
only recommended for very fresh paint. Pressures ranging from
3000-5000 psi along with turbo nozzles may be needed to even touch
this job.
I would only reccomend electric when you are working in a confined or
semi-ventilated area like a garage.
This is probably not the right venue to reccomend a specific product
so  try a google search with the term
grafitti removal + chemicals. You will find what you need to get the job done 
Sometimes renting a machine from a quality equipment rental company
can be a cost effective solution.

Randall Madon
www.The-Power-Washer-Advisor.com

There are various methods for removing spray paint from a sidewalk.
You can use either electric or gas power pressure washer. The most
important thing is that you utilize a pressure washer with at least
4000 PSI. However, just simply using high pressure will not do the
job. You will need to use a turbo rotary nozzle. This pressure washer
accessory is very effective because it multiplies ordinary pressure
into powerful scrubbing tool using a jet stream of rotating water. It
creates immense strikes that make it ideal for cleaning large, flat
surfaces such as sidewalks, driveways and patios. The way that rotary
nozzles operate is that they spin a zero degree water spray into a
circular motion creating a very strong scrubbing power. Never use this
type of nozzle on a wood, aluminum or brick. You will scar the surface
and damage the face. I highly recommend you first test it in the area
you want to clean. After you get the ?feel? for the pressure, continue
with power washing. Caution! Make sure the nozzle is fully attached at
the tip of the wand, because if it isn?t it can became a projectile
and hurt someone or damage the property.

For additional information on pressure washing tips go to <a
href="http://www.power-pressure-washers.htm">power pressure
washers</a> page

Another great tool for removing fresh paint is the sand blaster. This
piece of equipment is used in conjunction with the pressure washer.
The best is to use around 4000 PSI. The sand blaster consists of
bronze or aluminum body, tungsten carbide tip, blast head, clear tube
or a hose and a probe. The way it operates is that is combines high
pressure water with sand to remove tough dirt and deep stains. Besides
dry silica sand you can use baking soda or washed and dried river
sand.

Go to <a href="http://www.power-pressure-washers.com">
http://www.power-pressure-washers.com </a> page

I totally agree with aviationga that the best tools for removing any
type of paint is pressure washer sand blaster and/or rotary nozzle.
Here is the website where you can find these items with good quality
and inexpensive.


www.ultimatewasher.com

Go to Home Depot Tool Rentals, they have the chemicals in the rental
center for sale either separately or with the pressure washer rental.