I can't recommend a processor speed, but I can address the lightgun question.
Here's a little chart of the different light guns and which features each gun has:
http://jstookey.com/arcade/lightguns/I gathered this information from messages on this board. If you're using a standard resolution arcade monitor with the ArcadeVGA, then the Act Labs PC USB light guns are probably the ones you are after. You can use multiple lightguns with Windows XP using Mame Analog+ XP.
I'm not aware of there being a how-to. To run Mame Analog+ XP from the command line for multiple lightgun support, you would use:
mameanalogxp -lightgun -multimousexp policetr
And I think that you will probably have to configure which pointer devices (lightguns and mice) are mapped to each player using the -switchmice switch:
mameanalogxp -lightgun -switchmice -multimousexp policetr
Once the game is started, you might also need to remap the lightgun trigger & button to each player in Mame's tab menu.
I think that's everything, but I don't have multiple light guns to actually test the process out on.
Another important note to make is related to a discovery that Andy Warne at Ultimarc made. He discovered that if you use an arcade monitor and you splice the hsync and vsync wires together in your VGA cable hack, then the light gun won't work. When my lightgun didn't work on my arcade monitor, I added a wire to my monitor's output cable for the vsync, and wired it correctly to the video card, and only then did the gun work.
Here's a pic of what I added:
http://jstookey.com/arcade/WG_25k7191/WG_25K7191_output.JPG