The corei5 stock heat sink is garbage! You can't O.C. well with it. At stock I have had troubles with high temps and O.C. just makes it worse without voltage increases. Pick up better heatsink if you plan to get to 4GHz.
Nice build, I love my i5-750 build. I also really like the CPU cooler, it is not the highest rated but it is a very good performer and it also looks cool!