Should I buy faster memory?

Recently, many people with slightly elderly Athlon XP or Pentium 4 systems have started looking at upgrades. Memory upgrades, particularly.

Many of these systems were not built with DDR400, which is the fastest standard type of DDR memory. It's also the most widely available. The logical question then becomes whether DDR400 is a worthwhile investment.

The answer, as you might guess, is not perfectly straightforward. Primarily, it depends on your processor.

If you have an AMD Athlon XP, an Intel Pentium 4, an Intel Pentium D, a server version of either (Athlon MP, Xeon), or an older chip from either manufacturer (that is, virtually anything not on the list coming in a moment), it is a matter of Front Side Bus. For all of those processors, it is very desirable to have memory that can run in synchronous mode with the processor. To do this, you need memory capable of handling the real clock frequency of the front side bus your system uses. Helpfully, neither manufacturer is in the practice of mentioning this specification directly.

What they list instead is the frequency of transfers made on the bus. This is useful, and good for marketing purposes, but doesn't immediately give us the number we want. Late-model AMD and Intel processors use multi-pumped buses; they can complete more than one transfer in one actual clock cycle of the bus. But getting back to the numbers we want is simpler than you might fear. Look up the socket your processor uses on The Big Processor Guide. Read the entry on the table under "bus". If it says "double-pumped" divide the advertised number by 2. If it says "quad-pumped", divide by 4. Done. As a general rule, AMD's chips since the Athlon (and before those we'll get to in a moment) are double-pumped and Intel's chips since the Pentium 4 are quad-pumped.

Now you need to relate this speed to DDR speed. It's easy enough: multiply by 2. So for a double-pumped 333 MHz bus like the one many Athlon XP Bartons have, you would need DDR333. If you want to use the PC convention instead, multiply the DDR number by 8. DDR333 becomes PC2700.

If the memory you currently have is slower than this speed, chuck it and replace it with something faster. In the current market, you may as well go ahead with DDR400. If your current memory matches or beats this speed, keep it and add on. Buy something that is at least this speed. Again, as it stands right now you may as well buy DDR400. But bear in mind that buying faster than this minimum speed will offer no performance improvement.

Now, the aforementioned other list. If you have an Athlon 64, Opteron, Turion 64, or in general any AMD64 product, the above advice does not apply to you. Your memory controller has all sorts of voodoo powers, and paramount among these is the fact that it cares rather little about memory bandwidth. Athlon 64 systems with PC2100 can and do perform admirably. But they will benefit from faster memory up to DDR400.

For AMD64 processors, your decision is less clear-cut. Replacing what you already have with DDR400 will offer some performance improvement, but not generally as much as keeping the existing memory and adding in the memory you would have replaced it with. In general you should prioritize quantity over quality, as it were. As above, any memory you do buy may as well be DDR400; there is simply no compelling savings to slower stuff.

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