Generally, ATI's Catalyst Control Centre is all you need to manage your graphics card's settings. It even includes a rudimentary overclocking utility dubbed “ATI Overdrive”. The only limitation with ATI Overdrive is that clock speeds are limited by the card's BIOS, and usually the maximum clock speed you can attain with ATI Overdrive is pretty conservative.
One easy to way to overcome this is to get MSI's Afterburner overclocking utility. Developed in conjunction with RivaTuner, it is an extremely easy to use overclocking utility that lets you tweak core and memory clock speeds, as well as core voltage values and shader clock speeds on NVIDIA cards.
To go above the limit on Overdrive simply follow these simple steps:
Open MSI Afterburner, now you should be able to go above the limit offered by ATI Overdrive.
But of course, take heed when overclocking. Without going too much into details, you'll need to beef up on cooling and also ensure your card receives sufficient voltage. More on that in another post.
Specifications are not everything. It's what you do with what you have that matters.