Just upgraded my rig...
I added a second Raptor 74.5 GB and two more gigs of RAM, and hit it with Vista Ultimate 64-bit edition. I have an A8N SLI Premium board -- and it clocks down to 333 with four sticks of RAM. Thing is, it really doesn't matter.
Note also that DDR runs at tighter timings than DDR2, which means that DDR2 needs a faster clock to match DDR. DDR2 has more potential for improvement, but as things currently stand, it's no faster in practice.
Tom's Hardware did a pretty extensive investigation of RAM speeds a while back, and discovered that RAM clock and timings don't make much of a difference -- we're talking a few per cent here or there, and on most systems that don't run their CPU's at extreme clock speeds, the difference may not even be detectable.
Most games nowadays aren't CPU/RAM bottlenecked anyway; they're disk I/O bottlenecked and GPU bottlenecked. That means that more RAM will make for a nicer gaming experience with less stuttering due to disk I/O, even if you might experience a drop in frame rate on the rare occasions the game is CPU or RAM bottlenecked.
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http://www.tomshardware.com/2006/03/31/tight_timings_vs_high_clock_frequencies/ ]
Quote from the conclusion:
The bottom line is that as long as you have enough memory - preferably 2 GB - the extra money you pay for more memory speed would be better invested in a faster graphics card. And if you don't play games, then the CPU and hard drive offer more room for improvement than the memory.
I.e., chasing RAM speed numbers makes even less sense than chasing CPU clock speed numbers. If you really want to boost your system, buy a pair of fast disks for your system disk and RAID-0 them -- and then pay yourself silly buying a really good GPU.
