I am gathering the components required to do this as well(if i can ever convince my wife to give me the OK). i don't have any of the PS1 gun games, so i don't know how they are. i do however have Time Crisis 2, Time Crisis 3, Crisis Zone, and Vampire NIght. i haven't played crisis zone yet(just bought it a few days ago) but the others work well and are alot of fun. out of the ones i have, vampire night gets played the most because of the 2 player mode. Im not sure about crisis zone, but time crisis 2 and 3 are only 1 player...they do have a 2 player mode, but it requires two TVs, to PS2s and two copies of the game and some kind of special cable to link the ps2s together (im assuming it is a mini firewire cable, and this only works with the old style ps2 units)
the ps2 controller as a reload pedel is pretty cool too. i hacked an old ps2 controller to add a 1/8" mono minijack to one of the button inputs, so i could connect an old pedal i had lying around from my sega saturn gun.
like everyone esle said, you will pretty much need a CRT TV to get it up and running...preferably one with component video inputs.