My recommendation for any "which mobo should I buy?" is figure out your target price, and find out which Asus or Gigabyte models are available. I prefer Asus's UEFI to Gigabytes BIOS controls over EFI settings, so I bought an ATX size Asus P8P67 as soon as the fixed versions came on the market. Picking an Asus or Gigabyte at about the same price, should be about which one you prefer, based on looks, specs, and/or layout.
EATX is a bigger version of ATX, so I'd check your case before buying an EATX mobo.
Most 2500K CPUs will OC to 4 Ghz or higher, or so was claimed when I bought mine. I even had the CPU at 4Ghz with the stock cooler, then set it back to stock until I found a better cooler. Eventually I got a Corsair A70. I set it up with 2 fans connected to a spare Zalman fan speed dial to keep the noise down. I think any compatible heatsink with one or two 120mm fans works about the same. Watch out what kind of heatsink the RAM has. In my setup the "pull" fan covers the 1st RAM slot, and a large heatsink on certain RAM kits might interfere with the fan on the second RAM slot. Asus recommends using the 2nd and 4th slots with 2 sticks of RAM. I'm using Corsair RAM with the basic heat spreaders, but not heatsinks and I don't have this issue.