1: Anyone who is serious at gaming does so at low settings. Period. I still see some CS:S players using 1024x768, especially if they happen to have >120Hz CRT screens...If FPS is an issue, reduce settings.
2: CoD, like its spiritual predicessor CS:S, favors those with the fastest reactions and the best aim for headshots. No amount of equipment or training can offset that. I would know; my reaction speed has dropped over the years, and I'm below average in both CoD and CS:S now...[hence why I favor the more teamwork oriented Battlefield series, as tactics can often beat superior players, but I digress].
3: I always recommend the G15 to other gamers; the ability to tilt the middlemouse left and right offers VERY quick access buttons for the user. Immediatly avaliable are:
1: Left Mouse
2: Right Mouse
3: Middle Mouse
4: Side Button 1
5: Side Button 2
6: Mouse Wheel Up
7: Mouse Wheel Down
8: Middle Mouse Left
9: Middle Mouse Right
Plus two more buttons on the mouse. As someone who grew up with DOOM and never graduated to quake (and this, no WSAD skills; I still use the directional arrows), I find the G15 a godsend. A mouse (or any other gamepad) with lots of buttons instantly avaliable is huge.
4: Don't neglect sound. If you can't hear an enemy walking behind you at 10 paces, upgrade your sound setup. Using both a ASUS D2 and my G35 (depending on the game), its a HUGE advantage to hear that guy sneaking up on you, going for the knife kill...