Okay, so I'm having this problem, where input lag only occurs when playing FPS games such as Left 4 Dead. Other games such as MMORPGs and RTS games do not get input lag (well, not as much as FPS games).
This all happened after I switched from my old dual core, 2GB ram, 8800gt build. Now I'm using an integrated GPU so I'm not sure if the crappy GPU is the reason, but for sure I'm using the same LCD monitor.
I am planning on getting a new video card soon, so hopefully that is the case. Otherwise, what other reasons are there?
AMD Phenom II X4 955 3.2GHz
Artic Cooling Freezer 64 Pro
ASUS M4A785TD-V EVO
2x 2GB Patriot DDR3 1333
eVGA GTX 460 768MB
500GB Seagate Barracuda 7200RPM SATA
Cooler Master Silent Pro 600W
Windows 7 Pro 64-bit