Unless you could sell your existing build and recover a substantial enough return, moving to LGA1155 will be quite costly. LGA1366 stocks should get cheaper soon for clearance for the new LGA2011 socket, you may be in luck in that sense.
I don't know what the purpose of the build is but I will assume it is gaming and media based.
If you got money back and decide to go for a 1155 setup, consider;
ASUS P8P67 Pro Motherboard or P8Z68 V Pro if you need the Intel Smart Response Technology.
I5 2500K, it is $100 less than the 2600K and produces similar performance, like I said if you need the 4cores and 8 threads for a reason that I don't know about, the 2500K will be a wiser option.