How does RAM frequency and latency differ? How does it affect performance?

I have a question regarding two specifications of RAM memory, the amount of Mhz and the latency timings (example: 9-9-9-24).

Which one of the two is most important for system performance and why? Also what exactly is the difference between the two?

An article from Toms Hardware does a very good job at explaining RAM timings

• CAS, usually expanded as column address strobe (or sometimes as column address select), which refers to the column for some physical memory location in an array composed of columns and rows of capacitors used in dynamic random access memory (DRAM) modules (of which all three types of RAM in this guide are sub-types). CAS latency generally appears first in timing sequences for RAM and indicates the number of clock cycles that elapse between when the memory controller instructs the memory module to access a particular column in its current row, and when such access produces the data that resides there.
• Trcd or tRCD, usually expanded as RAS to CAS delay, where RAS is expanded as row address strobe, where R refers to the row for a physical memory location in an array composed of columns and rows of capacitors used for DRAM modules. This value specifies the number of clock cycles between a Row Address Strobe (RAS) and a CAS, and represents the row address to column address delay for a memory module.
• Trp or tRP, usually expanded as RAS precharge, which represents the number of clock cycles required to end access to the current row of memory, and commence access to the next row of memory, so that tRP = time for row precharge.
• tRAS or Tras, usually expanded as RAS access time measured by the number of clock cycles needed to access a certain row of data in DRAM between the initial data request and the precharge command required to commence the next memory access. By definition, tRAS must be greater than or equal to the CAS plus the tRCD, plus an additional two cycles, to leave time for accesses to complete, as they read or write multiple bits of memory, which DDR (2 bits), DDR2 (4 bits), and DDR3 (8 bits) all do in lesser or greater numbers.

RAM memory timings usually appear as sequences of four numbers separated by dashes, as in 5-5-5-15. This indicates that the CAS, tRCD and tRP values are all equal to five clock cycles, and that the tRAS value is equal to 15 clock cycles. The smaller the numbers that appear in these sequences, the tighter memory timings are said to be. Likewise, larger numbers are said to indicate looser timings. Simply put, lower latency costs more, tighter timings cost more and the combination of the two costs the most where memory is concerned.

Source

I would go with the tighter timings they tend to be faster and the 166mhz difference will not even be noticeable. In fact they are both so close that you would not notice any difference in speed. The one with the higher Mhz rating might be best for over clocking since it will give you a little head room since modern Intel and all AMD chips have integrated Memory controllers so as you up the FSB the you will also be over clocking the RAM. But again this is all personal preference depending on what you will do with your system. Either one will be fine if you are just building a stock system that you don't plan on overclocking.