-----Original Message-----
Ideally, it's more important and advantageous that the CPU FSB and memory
run at the same speed. IOW, the FSB and memory "clocks" are synchronous.
Since you didn't mention anything about your processor, it's hard to be
precise here, I can only speak in general terms.
Suppose you have an Athlon XP 2600 "Barton" 333MHz (DDR), that's 166MHz
"clock". If you installed only PC2100, which is 133MHz "clock", then
(assuming no overclocking) you'd be running the CPU FSB @ 166MHz, but the
memory at 133MHz, i.e., "asynchronously". As a general rule, computers run
more efficiently when the FSB and memory clocks are "in sync". The Barton
would be better matched to the PC2700 (166MHz "clock"). On the other hand,
suppose you had an Intel P4 533MHz (133MHz "clock") CPU. In this case, the
PC2100 (133MHz "clock") would be a better match, since it runs synchronously
w/ the CPU FSB.
That's why you really can't discuss memory selection outside the context of
the CPU (or more precisely, the FSB). There's a relationship between the
CPU FSB and memory that's relevant too, not just the pure speed potential of
the memory. Trying to discuss the best configuration of your PC w/o regard
to both, it like trying to get a quality picture w/ your camera and
discussing only the lens aperture or shutter speed. It takes BOTH to get
the right exposure, not just one or the other.
Some people do claim better performance when the FSB and memory are
asynchronous, but lots of empirical testings has shown that in most cases,
unless the differences are substantial, the benefits are so slight as to be
hardly worth the effort. Here's an example of a recent examination of
running two identical configurations, one 250/250 (sync), vs. 250/200
(async):
http://www.cluboverclocker.com/reviews/memory/corsair/xms3 200LLPRO/index.htm
The benefits of 250/250 (PC4000) vs. 250/200 (PC3200) are so slight, who
could be bothered. Of course, the fact that latencies are a real problem at
these speeds contributes significantly to the lack of performance gains,
most PC2700 and PC2100 could probably run w/ closer timings, and so there
might be a slightly better case for the PC2700 in an asynchronous
configuration.
Of course, some people over-buy memory (PC2700 when PC2100 will do) if they
intend to overclock the CPU FSB. This way, as they increase the CPU FSB,
the memory remains underclocked and doesn't become a bottleneck to
successful OC'ing of the FSB. Whether this is a concern or interest to you
is beyond this discussion, I just want to complete the picture, since there
are people who *do* buy faster memory for such reasons.
As an aside, anytime you *mix* memory speeds (PC2700 and PC2100), it's
doubtful your motherboard will support the faster of the two by default (By
SPD). If it will support the faster speed of the two, and that's iffy, it
would only happen if you *manually* set the Memory Freq (and timings) in the
BIOS. Most any motherboard is going to default to the most conservative
configuration.
But for most mere mortals, you match the CPU FSB and memory speed for
typical, normal, everyday operations.
HTH
Jim
therefore, the
lesser