Memory Timings Benchmark Impact
The tests were performed with a 290 MHz HTT base speed and a 2/3 memory speed divider, resulting in 193 MHz memory clock - this equals DDR386. This speed is almost as comparable to 290 MHz (DDR580) as DDR400 is to DDR600.
The different timing settings produce nice scaling results in SuperPI, whereas in 3DMark01 the middle three results pretty much just represent normal benchmark run variations. Everest results look normal, and SiSoft Sandra doesn't really seem to care about timings.
What's of most interest here is to compare the most relaxed timings to the tightest timing settings. When keeping the timings and changing the system memory clock frequency from 200 to 290 MHz, we saw 5.2% and 3.5% better performance in SuperPI and 3DMark01. Now, when we're keeping the clock frequency at 193 MHz and changing the timings from CL3.0-4-4-7 to CL2.0-2-2-6 - settings you probably would use when running DDR600 and DDR400 respectively - we see a difference of 5.1% and 5.7%. Interestingly enough, the difference between the best and the worst times in SuperPI is 19 seconds in both cases.
It seems as if 3DMark01, at least, reacts worse to relaxed timings than to slow system memory clock frequencies. In any case, CL2.0-2-2-6 @ DDR386 got outperformed by CL2.5-4-3-7 @ DDR580 in all of our tests. The synchronous HTT/memory speeds when using DDR580 might play a role as well.
These timings can only really be reached at that speed when using 2x 512 or 2x 256 MB modules. For 2x 1 GB modules, if you can find any that actually run at these tight settings, you will have to use CL3.0-3-3-7, or more likely, CL3.0-4-4-8.