CFM determine how much airflow the fans will push. You will want something in the 2500 and up range depending on your needs...probably closer to the 3000 CFM range.

I have a BeCool 2760 CFM dual 11" fan on my radiator with a 20 amp draw (measured) and it keeps my big block stroker at 185-190 degrees at stand still on a cool day. This is on the low end of the factory temp gauges.

I don't think that everyone needs to these monster 30-40+ amp fans...in many cases that can be a 'BandAid' for a deficient cooling system or other engine issue. Lots of things can cause overheating besides not enough airflow. Not to say it's a bad idea to have extra cooling...just may not be necessary.

Also say goodbye to the a factory alternator with a 30 amp+ draw.

However if the motor is running cool when moving but the minute you slow down, we have overheating then it could be an airflow issue. I would look at the CFM ratings on the fans and compare to what I said above. Fan shrouds that fit tightly over the radiator also make a huge difference to make sure all the airflow is 'funneled' through the radiator.

And yes, there is a huge difference between the quality of the $50 cheapo fans and the more expensive units. I would expect to pay $250-$300+ for a quality fan/shroud setup (minus the wiring/relays/etc.)

Good Luck!