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.
