Asus 1155 P8H61 ML X3 for gaming

Solution
Well, if you are wanting to get that i5, then you have to keep the board you have now as the i5 won't fit on that newer board. (Ivy Bridge not compatible with Haswell boards)
15-2500k will fit yes, but you are limited in what you can achieve with overclocking since that board does not have a Z-series chipset. All the necessary goodies for overclocking will be locked down.
Since the i5-3750 is a better chip than the 2500k, rather stick with that one.
What you have now should suit you just fine if you not wanting Ultra or high graphics settings.
Look at my set up in my signature. Plays everything I need it to at High settings or more at 1600x900. (Screen limitation)