The ASRock Z68 Extreme3 Gen3 is considered the optimal budget Z68 Motherboard. Many have over clocked their i5-2500K to 4.5GHz with relative ease.
Reaching 5GHz would depend on variety of factors including your CPU, motherboard, and cooling.
A solid choice for higher over clocking is the ASUS P8Z68-V PRO/GEN3 but it will cost a little more.