Eliminating slow downs...

I realize that i need to turn something down (or off), but im not exactly sure what graphics and audio modes take up the most resources. I'm running pj64 with a fullscreen mode at 1024X768, triple buffer, everything enabled (including anti-aliasing) and would like to know what exactly is taking up the most resources cause this mode looks pretty decent. Im getting a steady 60+ frames per second but things will slow down considerably in a fire fight in perfect dark.

Comp stats that matter here:
AMD 1700+
512 MB RAM
ATi Raedon 64 MB DDR VIVO
Soundblaster Live!
Windows XP

yea, but i remember playing it on the 64 and didnt get the slow downs. this isnt just for a big gun fight, just two guys will slow this down