Well, you need to enter BIOS(by hitting del key while POST is going on). Sometimes the key is different..... may not always be DEL.
Anyway once you enter BIOS, you just browse around the settings until you come across a setting called "increase FSB". This should be set to 200mhz as default value.
Increase it slightly say 210mhz.
Reboot.
Run CPUZ. Download it from here
http://www.cpuid.com/download/cpu-z-132.zip
You will see that the speed shown will be 210 x 11 = 2310. this will be a 110mhz overclock over the stock speed of 2200mhz.
Run Prime 95 for abt 24 hrs to check for stability. If Prime 95 runs for this time without giving errors, then it means your CPU is stable at this higher speed.
Keep increaseing the FSB slowly until your computer become unstable.
Then increase the vcore slightly. In steps of the minimum increase allowed by your mobo. Then repeat the process...
Note that above 1.55V is not recommended for the proccies. And keep a watch on the temps of the CPU. If it gets more than 50 deg C on load, it is a bad sign
http://www.techenclave.com/forums/collection-a64-overclocking-tips-and-guides-4849.html
Read the above thread and visit the links for more info on overclocking