Overclocking a VGA card will only benefit you as far as upping the frame rates in 3D applications, mainly games. When overclocking components we don't generally overclock graphics cards, because the performance impact is minimal in general applications, and the thermal limits of GPUs don't really provide a lot of head room for overclocking.

For ATI, use the ATI Tray Tool, while for NVIDIA we recommend Riva Tuner. When overclocking the core you can use 5 MHz increments initially, but after an overclock of 25 MHz or so, tone down to 1 MHz increments.

When overclocking memory be a little more careful as most graphics cards' memory doesn't really have much headroom. Use 1 MHz increments. Make sure to run a couple of 3D benchmarks like 3D Mark 2006, or loop it a couple of times to veritY stability.