Scrypt is very dependent on memory speed so the core/memery ratio really is the most important criteria. Of course the best way to find the ideal ratio is if someone has already done the work and has found posted it somewhere!

Generally, the core should be roughly 0.7 the speed of the memory. So with a core of 1000MHz that means ~1428MHz. On 7950s I've found these rations to work 1022/1244 or 1100/1500.

For experimenting with rations, I found cgminer the best since unlike when using the GUI front-end (GUIMiner - which anyway uses cgminer), you can change the GPU settings dynamically in the program without having to disconnect. So, just hit