As you can see from my Sig below, we have similar rigs.
Your Mobo should have come with a CD with a suite of Asus programs. Within one of the programs is an application which automatically OC's the CPU to the highest stable setting. It's very easy compared to "the old days" (I started OC'ing with an AMD XP1700+). You can do it through the BIOS, but the Asus utility is good and simple. Edit: I'm referring to the same program as Raw Kryptonite.
I'm sorry I'm Not more specific ATM but look for the Asus CD and MoBo manual and install the utility. If you still have trouble post back and I'll pull up all the info and answer questions
BTW, I got to 4.8GHz with the STOCK Intel fan! The 2500k is a great OCer and yours should get to 4.3 easily.