what your talking about is called nkro (n key roll over) what mean is, is on most keyboards you can push up to 3 keys simultaneously and they register (which is called 2kro). some gaming keyboards are nkro while using ps2 connection which means you can push as many keys as you want at the same time and all will register, now these keyboards when used the a usb connection can only have 6kro+keypad (6 keys + keypad).
as for keyboard suggestion lets see.....
Steelseries 6gv2 (mechanical full NKRO)- my prefered keyboard, it is most excellent. cherry MX black switches.
Steel Series 6gv2
Razer black widow (mechanical 2kro) heard both ways about this keyboard, many have said it is better quality than most razer products. cherry MX blue switches.
Razer Black Widow Black Widow Ultimate
Deck Legend (mechanical NKRO) very nice looking keyboard if backlighting and NKRO is a must. cherry MX black or brown switches.
legend or if you dont want a keypad
82 key
Those would be my suggestions but its all up to you and your own research you do

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