On the table below you see all official speeds for SDRAM, DDR-SDRAM and DDR2. DDR and DDR2 transfer two data per clock cycle, so its rated clock isn't its real clock, as it occurs with the external bus of CPUs from AMD. We listed DDR2-667 and DDR2-800 just as a reference, since these memory types are not available on the market yet.

Memory	Tecnology	Rated Clock	Real Clock	Maximum Transfer Rate
PC66	SDRAM	66 MHz	66 MHz	533 MB/s
PC100	SDRAM	100 MHz	100 MHz	800 MB/s
PC133	SDRAM	133 MHz	133 MHz	1,066 MB/s
DDR200	DDR-SDRAM	200 MHz	100 MHz	1,600 MB/s
DDR266	DDR-SDRAM	266 MHz	133 MHz	2,100 MB/s
DDR333	DDR-SDRAM	333 MHz	166 MHz	2,700 MB/s
DDR400	DDR-SDRAM	400 MHz	200 MHz	3,200 MB/s
DDR2-400	DDR2-SDRAM	400 MHz	200 MHz	3,200 MB/s
DDR2-533	DDR2-SDRAM	533 MHz	266 MHz	4,264 MB/s
DDR2-667	DDR2-SDRAM	667 MHz	333 MHz	5,336 MB/s
DDR2-800	DDR2-SDRAM	800 MHz	400 MHz	6,400 MB/s

As we've been saying, in order to achieve the top performance your PC is capable of giving you, you should match the memory speed with the CPU speed (it is ok if your memory is faster than your CPU external bus, the problem is the other way around).

Actually, if you have an AMD CPU you won't have problems. For instance, if you have an Athlon XP with 400 MHz (3,200 MB/s transfer rate) external bus just install DDR400 memories on your computer and it will work great since both the CPU and the memory will be running at the same speed grade (3,200 MB/s). You can use DDR Dual Channel to improve the performance, anyway, as we will be talking about on the next page.

Intel processors are a problem. You don't even need to have a state-of-the-art PC to have memory bottleneck. The 2.4 GHz Pentium 4 from Figure 2 would need a 4,264 MB/s memory and from the table above you can see that only DDR2-533 can provide that. If you have a newer CPU with 800 MHz external clock rate, you would need DDR2-800 memories on your PC – a memory type that was not even lauched yet!

That's why Intel is pushing DDR Dual Channel. With this technique memory bandwidth is doubled and you can match the CPU speed with memory speed.

So, specially with Intel processors, if you don't use Dual Channel Memories your PC will achieve a performance lower than it is capable of.