I would get a Intel i7-4790k it's much better than the cpu you have now and would work well with 2 Titan X's. The Intel i7-4790k is about £250 but is what most gamers use. If you don't have the money, just stick with the cpu you have now and overclock it to about 4.4Ghz. It will work just fine with the 2 Titan X's and you don't need to upgrade or overclock, but if you want more performance either upgrade or overclock.