In CS2? The speed difference between say, single and dual core processors of the same clock speed when you use filters and resize large images is quite significant. But that is provided you have at least 1GB RAM to begin with.
So for good performance in CS, you should spend more on RAM first, followed by a faster or dual core processor. If you can afford to have both, that is the best setup, ie. 2GB RAM and dual core processor with high clockspeed. If not, get 2GB RAM first for the best performance your $$$ can afford.