The easy solution to any memory debate concerning the latest processors might sound simple : just buy DDR2-800 (PC2-6400). Consider that all socket AM2 processors are designed for it, and it’s supported by most current LGA775 motherboards as well. But it’s also expensive, and most DDR2-800 requires the user to adjust motherboard BIOS settings to achieve its proper speed.
DDR2-800 typically requires a nonstandard 1.9 V or higher (up to 2.2 V) simply to run at its rated speed. Most boards will automatically select 1.8 V, which will cause DDR2-800 modules to be detected at a slower speed that the manufacturer has determined to be stable at the lower voltage. (This explains why neophytes have been seen rushing, upset and confused, to the TG Forumz because their DDR2-800 has been detected as DDR2-533 or DDR2-667...)
Corsair and nVidia have come up with a solution, in the form of extended SPD values ; these are branded as Enhanced Performance Profiles (EPP) and SLI Memory, respectively. These values tell the motherboard what "overclock" settings to use, but require a compatible board, most likely with an nVidia chipset. The technology has been left open for others to use, and OCZ recently announced its new products as well.
The bottom line is that DDR2-800 is still expensive, and furthermore, it is not really required. Budget builders will note that DDR2-400 also aligns exactly with our AM2 chart, and speeds such as DDR2-533 and DDR2-667 will usually operate at nothing worse than a slight underclock.
On the Intel side, DDR2-533 in dual-channel mode is an exact match in both bandwidth and clock speed to its current FSB1066, and DDR2-667 in dual-channel mode exactly matches the next-generation FSB1333. Slightly better performance numbers are often achieved using faster-than-matching memory, but this is mostly due to decreased latency as cycle times become shorter.
PC2-4300 is fast enough for most DDR2 systems, PC2-5300 will provide a slight performance gain, and PC2-6400, while nice, is not entirely necessary. But what about low-cost systems that still use outdated "regular DDR" (a.k.a. DDR-1) memory ?
PC-3200 (DDR-400) is so common that 512 MB modules can be found for as little as $10...after a few mail-in rebates. Because it’s so cheap, there’s no need to choose slower PC-2700 in any new system build, even if the processor runs at a slower bus speed.
In terms of memory quantity, Tom’s Hardware Guide recommends a minimum of 512 MB for the cheapest single-task Windows XP equipped systems, 1 GB for normal Windows XP systems, and 2 GB for Windows XP performance systems. With Windows Vista quickly becoming a reality, those who can afford 2 GB now are best advised to choose this as a good starting point !
A final note : buying recognizable name-brand modules with lifetime warranties from reputable venders is good insurance against "unexplained" system instability.