In my opinion the PSR500 is already so very senstive...especially in RF rich areas..cell towers, urban, big suburbs...that I already get some front end overload at times. I would never buy one for mine. I think it would just create intense feedback on me.
I use a Hamtronics amp on my base 800 antenna and it does a great job in pulling in 800 signals. However I am in the country, buried in the woods with high trees and high house so I don't worry about front end overload. But, as Statevillan said I think in most cases an amp will overload the front end of the PSR500. I too used the GRE amp on my Pro96 but have not used it on the PSR500 in a mobile environment. I play with the squelch and ATT to improve reception in a mobile mode.
I was having problems with monitoring a 800mhz system in arizona. I bought the super amp and it greatly improved my reception of this system. Ofcourse the gre scanners are more sensitive and systems that are very close to me overload but luckily that amp has a bypass to easily turn of amplification. Not to mention you can adjust the gain to work all the way around.....definetly reccomend it.