for Photoshop, Memory CAPACITY (1-2GB+), not speed, is king. You could have a 90GHz processor, but if you only have 256MB of RAM, it won't make a difference.
for gaming, your graphics card often has the biggest influence.
There are two types of memory speed so to speak. You have frequency ("DDR400", "PC3200", etc) and timings ("2-3-3-10, 1T", etc [See: 'Performance' RAM])
Timings, for the most part, have very little real world impact. As far as frequency goes, as long as you aren't buying PC2100 memory for a system that needs PC3200, you'll be fine.
But to answer your question, a faster CPU will have a larger impact.
- JaAG