What is better performance wise with i7 2600k: 1x4GB or 2x2GB RAM?

Solution 1:

Quoting here:

All Sandy Bridge designs also incorporate a dual-channel DDR3 controller, suggesting that the triple-channel system used by the LGA 1366 platform is being sidelined.

“We’ve found a very high percentage of clients will only tolerate two memory channels,” explained Opher Kahn, senior principal engineer for Sandy Bridge. “OEMs wanting to build for three channels need to use chips that aren’t based on this design. It’s a small market that we’re not going to separate out of this chip design.”

So it's a dual channel architecture and you should get better performance with two RAM modules.

Solution 2:

2x2GB will provide better performance in all applications because it will allow you to place an equal amount of RAM on each channel of the memory controller. However, 1x4GB will leave more slots available for future expansion.