Xigmatek Gaia interfere with the case fan (only pull fan for CPU cooler is possible but not push fan). Being just pull only, the Xigmatek Gaia is insufficient to cool down i7-2600K, I ended up using Cooler Master Vortex Plus. At room temperature of 24C, the CPU temperature is running great now...
So I guess with Noctua NH-U12P only one fan could be installed too. Hmmmm....
Maybe this one will fit (Zalman CNPS9900NT): http://www.zalman.co.kr/ENG/product/Product_Read.asp?idx=354