You can try using a program called CoolBits.
Look there and you should see a link to download Coolbits. It extracts and places itself into your system registry so be aware of that.
CoolBits imo is far more easier to understand and use than rivatuner, although rivatuner is far more powerful. Use CoolBits to increase core clock speeds / memory clock speeds of your videocard, use rivatuner to do advanced overclocking like unlocking extra pipelines on your video card [read up on it when you've finished overclocking the basics].
Read in the link I sent you, it gives you tips on how to overclock your videocard, also signs you can check on when you've gone a bit too far with your clocking.