Not to belabor the gaming aspect. But according to the people who pay money to determine reaction times (like insurance companies), it takes a half second to react to visual stimuli (longer if complex: twitch shooting friendly fire) and another half second plus for motor nerve (muscle) response. The "subliminal ads flashed for 1/60th of a frame" thing had studies discrediting it. Some bars have "reaction" testers, its funny to watch people who think they're fast.
Network latency is a killer. Gaming on a fast network like a LAN party or on Microsoft campus is great but the trend is video and mp3 (big file) downloads at the expense of connection time, Clearwire being worse than the old AOL dial-up (proxy caching).
Your video card must translate a digital code to analog to go over the vga cable, the LCD display recieving a VGA signal must translate it to digital, then wait for the portion of the scan with the video update to show up. LCD displays are actually 4 to 36 (more on larger displays) controllers that refresh their own section of screen. Theoretically, like with mpegs, you only have to send the digital data that changed. Sony et al have TVs that run off 802.11 and I've installed presentation panels that ran on USB 1.1.
I've worked on systems for people who actually make money from their computers: CAD operators, stockbrokers, PCB board testers, Medical staff, etc (Lawyers, Realtors,...). and CPU speed, esp floating point, was important for rendering, video card could be mediocre as long as its driver didn't crash.