what kind of frame rates are you talking about? because one of my computers is a:
P4 2.4 ghz
1.5 gig DDR ram
80 gig hard drive
ATi 9600 Pro - just changed to a Ge-force 5500 because i ruined the ATi due to overclocking problems
and i average only about 30-40 fps at the best.. down to 20 and below during firefights.. so i think its your rig thats causing you to lag because face it, its not that great of a computer.. you may just be noticing things you didnt notice before because it is magnified on the 17' monitor