It largely depends on the type of applications you are likely to be running and also the price difference between the two and how much you are willing to spend. In general the fastest performer in single core apps/games would be the CPU with the latest architecture and the highest clock speed. However, any degree of multi-tasking will benefit from a multi-core CPU. If you are simply browsing the web or doing office tasks then a dual-core CPU would be ample. I have a machine here which my younger brother uses for web-browsing and his homework and that still has a 3GHz P4 but it performs well enough for his needs. However, if you're doing heavy multi-tasking or run applications that will benefit from multiple threads then you may need more cores.
I'd say a 532MHz clock speed would probably be noticeable in some games, but the graphics card would be more important. If you wanted to know, the fastest CPU for single core applications and games would probably be the Core i5 670.