1) To physically attach three monitors with current Nvidia cards, you will need two cards. AMD eyefinity can with effort, attach three. But one card may not be sufficient for three monitors.
2) Most games play on one monitor only, but this will change, I think.
3) In general, I like a single card when it will do the job. Power requirements are lower and cheaper. Also, some games do not scale well with multiple gpu's.
At upgrade time, you will have options. A second card, or sell and get the next best thing. The top end cards seem to hold value longer.
4) For the best gaming, I think a single 30" 2560 x 1600 monitor is best. But--- they cost about $1000. They contain about the same number of pixels as a pair of 1080P monitors.
5) For a single 2560 x 1600 monitor, a GTX580 will run fine. That is what I use, along with a secondary monitor. I recommend it.
6) Middle of March should launch the GTX590, a dual gpu card that can handle 3 monitors.
7) 27" monitors will be 1920 x 1200 or 1920 x 1080(1080P) resolution.
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You are getting a 2600K to be able to overclock. A 2500K should be able to overclock just as high. The Hyperthreading is all but useless for games.
I suggest you save $100 and put it towards a great graphics card. If the $100 is not that important to you, get the 2600K.
Go for a 8gb kit of 2 x 4gb. Your P67 motherboard will have 4 ram slots.
Look for the Intel 120gb 510 SSD due to launch on March 1. It should be faster. I don't know if it will be worth the $280 list price.