Memory

The C7 is slightly faster than the C9. Also, Intel's aboslute max spec for RAM on the P55 boards is 1.65V, so I would stay away from anything over that (in other words, don't get the Dominator's that run at 1.8V).

One final word of advice - if you are an overclocker the RAM you have picked out is fine. If not, you will not be able to run your RAM at 1600 MHz. With an i5 750 the faster you can hit without overclocking or making other trade-offs is 1333 MHz. That RAM will still work fine, it will just run at 1333 MHz. If it is similarly priced to 1333 CL 7 RAM then go ahead and get the CMX 1600s you picked out. But consider 1333 RAM instead if it will save you a decent chunk of change.

When it comes to the i-series chips, Intel says do volt higher than 1.65v or you may damage your CPU. That being said, choosing the CMX4GX3M2A1600C7 7-8-7-20 1.65V, would be best.