Videocards usually offer very little room for overclocking. Probably the best shot would be to buy a 'golden sample' or 'OC' version straight from the factory.
Overclocking the vc will create more heat, this will in turn create higher noise levels from the fan which is typically very noisy in vc's. Third party coolers are rare and expensive.
I have overclocked my videocard but then again I have a fully watercooled dual radiator system to handle the heat. And even then the overclock available from the vc is minimal.