The Divider Problem
The DDR2 memory speeds we set, such as 736 MHz for the Athlon 64 X2 4400+, weren't chosen at random, but were assigned by the CPU.
Looking for a moment at the operation of the DDR1 interface on the old Socket 939, we see that the processor converts the CPU clock speed using a divider, in order to come up with the correct speed to use for addressing the memory. Internal memory interfaces have used DDR400 (200 MHz) since they were invented.
Athlon 64 X2 4200+: 2200 MHz / 11 = 200 MHz (DDR400)
Athlon 64 X2 3200+: 2000 MHz / 10 = 200 MHz (DDR400)
For this reason, AMD only sells CPUs with even clock speeds, which are divisible by 200 MHz, in its range.
With the switch to DDR2, AMD runs up against a problem: DDR2-800 has a clock speed of 400 MHz and can therefore no longer be divided by all previously envisaged CPU clock speeds. And the highest speed possible is not divisible by 400.
But how does the CPU react if a memory timing is set which cannot be divided by the CPU clock speed?
AMD came up with something smart here: if a divider is produced that exceeds the next possible JEDEC-compliant standard memory clock (400, 533, 667, 800), the next lower divider is selected automatically.
Athlon 64 X2 4800+: 2400 MHz / 6 = 400 MHz (DDR2-800)
Athlon 64 X2 4000+: 2000 MHz / 5 = 400 MHz (DDR2-800)
Athlon 64 X2 5000+: 2600 MHz / 7 = 371 MHz (DDR2-742)
Athlon 64 X2 4400+: 2200 MHz / 6 = 366 MHz (DDR2-733)
This produces highly curious memory clocks like DDR2-742 and DDR2-733. The automatic selection of the divider cannot be deactivated or influenced.
We therefore advise the ambitious user to grab a pocket calculator before making a purchase, and check whether the divider for the CPU tallies or not. It may be that the dependence of the memory interface on clock speed means that, despite its 200 MHz higher speed, a CPU may actually be slower when running some applications. For example:
Athlon 64 X2 4200+: 2200 MHz with DDR2-733
Athlon 64 X2 4000+: 2000 MHz with DDR2-800
If you now consider the fact that the lower memory clock speed already forced on the 2200 MHz CPU is compounded by a cache merely half the size of the less expensive 4000+ at 2000 MHz, you have to ask yourself: who can really understand all of this?
An overview of all possible memory configurations at standard clock speeds
The memory clock speeds in red are not standard and are therefore rounded down, resulting in a performance loss.