i5 2500k Overclocking issues

4.5 is very attainable. Basically, the best way that I can think to start helping you is finding your voltage starting point.

Set your Bios to fail-safe defaults and then restart.
Now, when I first go in after this, I like to turn off the full screen splash so that you can see what your proc is posting at.

In the M.I.T. (Motherboard Intelligent Tweaker) on the first page it is going to show you your current Vcore. That is a good starting point (write this down).

Boot into windows and use Real Temp (3.67) and click the button that is in the Top Right until you see VID. This will show your Idle VID, so you want to make sure you have Prime 95 running, and you will get the load VID. This gives you an idea of what your CPU wants at this clock speed under full load. You can use Gigabyte Easytune 6 to view that actual voltages being supplied to the Chip.

Once you have all of this information, restart the PC, head back into the bios. Set a 40x multiplier and repeat what you just did for stock clocks. This is going to give you an idea of how much voltage is changing for the clocks. (This is all being done with bios cpu voltages on AUTO)

Once you have an idea of what the voltage trends are, you can take the CPU off of auto voltage and then manually input your Vcore. Remember that all chips are different so take this with a grain of salt, but to give you an idea of what I am working with...

4.6ghz 1.33v (I use offset voltage)
PLL voltage 1.65v
VCC 1.125V
DRAM Voltage 1.525V

Idle Temps 29c
Load Temps 64c

I hope this helps you man.