Generally if you have a crappy heatsink and fan you would replace them with a better HSF before adding heatsinks/fans in other locations.

Also, you might find that the original heatsink isn't even seated poperly on the GPU, so at the very least you should try to get it off and have a look at what's underneath. Usually people find that the heatsinks are stuck to the gpu with either too much thermal paste, a crappy thermal pad, nothing at all or that the heatsink isn't flat.

RivaTuner is probably the sofware program to start of with, but make sure you don't push it too far. The key to oc'ing video cards is small 1-2Mhz steps then test for stability.