GHz is not a measure of performance.
For example, if intel had kept with their Pentium 4 architecture and reduced heat and somehow managed to add cores and ended up with a 10GHz quad core pentium 4, it would not be as fast a phenom II x4 at 3.4GHz, not even close.
You have to take into account a few things with raw performance per core: The clockspeed, and how much work it does per clock.
The Phenom II does less work per Hz than the icore architecture. So it needs to run at a higher frequency in order to stay on par.
Expect the newer Bulldozer cores to run at a lower frequency, but be assured they have a higher IPC (instructions per cycle) and therefore will be faster. (so John Frueh from AMD server marketing says in a comment on his blog).
This is the same reason why AMD had their "performance rating" back in ~2003-2006. Their AMD Athlon 64 3200+ was about the same or better performance-wise than the pentium 4 at 3.2GHz but it was a 2.0GHz part.
Cache is something a bit different. Cache helps in some, but not all, applications. It basically keeps a small amount of memory on the CPU die itself so that the CPU will not have to access the RAM as often.