if you are a gamer, SLI/CF is the single most cost effective way to improve your in-game experience. This should be one of your primary concerns when building a system (along with monitor setup)
How many cards will you put into the system initially?
*will you replace these cards over time or add new cards as they decrease in price?
*will your motherboard/cpu support your desired end state (3-way and 4-way SLI are not possible on the maximus V formula for example)
*do you have enough power to support your cpu and SLI'd cards?
*do you intend to overclock your cpu or graphics?
*do you want a future proof system that can handle the next generation of games without needing to upgrade?
*how much of your machine do you expect to 'migrate' into your next build?
Answering these questions first will guide the rest of your build decisions as far as platform (cpu, motherboard, ram, and cooling)
Motherboard Asus ROG Rampage IV Formula x79
CPU Intel i7 4820k Ivy Bridge-E @ 4.6ghz
GPU(s) 3 WAY SLI -- EVGA GTX 680 SC Signature (2GB)
Displays 5140x1050 @ 120hz using 3 x Samsung 2233RZ
RAM 16 GB 2133mhz G.Skill RipJaws Z
PSU LEPA G Series G1600-MA
Case Cooler Master Cosmos II Ultra
Storage Vertex 3 256 (Games), Intel 320 Series 128 (OS), + 4TB HDD