You should have no problems with overclocking. It's just a matter of taking your time and not exceeding the limit imposed by the components in your computer (even though others with identical computers might manage more than you).
First of all, and this is very important before you attempt to overclock: Is your system 100% stable at stock speeds? If not, you need to address those issues first. Please post back if this is the case, and we'll endeavour to get your system running stably.
I've found this video that might help you:
You should have no problem with overclocking your graphics cards. This is done through the Catalyst Control Center
. Select Performance
and then click on AMD Overdrive
. When you have accepted the warning, you can proceed. This is what I have achieved with mine (both cards set identically):