It's not over

It's not over

If we weren't FiringSquad, we probably would have ended the article there and patted ourselves on the back. Most reviews on the net test 2x512MB memory setup or 2x256MB. It's cheap, popular, and certainly a recommended budget configuration. But this is all about high-end systems right? What if you want more than 2 DIMMs? That's where things get interesting.

Adding extra memory to your PC can slow it down

Athlon 64

Although Athlon64 motherboards ship with 3 DIMM slots, they can only run 2 DIMMS (4 banks) at full speed. This limits you an Athlon64 system to 2x256MB (two single bank), 2x512 (two double bank), or 2x1024 MB (two high-density double bank) memory configurations. Should you go with 3x256MB, your memory speed will drop to DDR333 and going with 3x512MB of RAM will drop you all the way down to DDR200 (PC1600)!

Registered Memory on Athlon 64 FX and Opteron

On the Athlon 64 FX and Opteron CPUs, there really isn't any difference in performance with additional DIMMs. Those CPUs are designed to handle 4 DIMMs (8 banks) without any problems, and since it's registered, adding DIMMs does not increase latency - you've already paid the toll at the register. Simply put, the registered DIMM design allows you to add additional memory to the system without adverse affecting system performance.

And Intel?

The Intel i865PE Springdale and i875P Canterwood platforms claim to preserve full DDR400 clock speeds, but will add additional latency with greater than two DIMMS. Those of you with careful reading skills will notice that I used the word "claim."

Lost in Translation? Here's the summary

On the Athlon64 and Pentium4 i875 platforms, the more memory you add, the slower the memory performance. In the case of the Athlon64, the drop occurs with a drop to 1.6GB/s bandwidth, half its peak. It can transfer less data for the same amount of time. In the case of the Pentium4, the performance drop is supposed to be an increase in latency, but maintains the DDR400 bandwidth of 3.2GB/sec. It's top speed is the same, but has slower acceleration.

What we are saying is that adding extra memory can reduce system performance if it is unused. You won't believe me without the proof, so let's take a look at the benchmarks again when running additional RAM.