After perusing this article, I've decided on the XMS 2. 800 seems to be the limit of usefulness for moderate overclocking (plus I don't want to cough up $300+ for memory).
As for the timings, is the $20 premium worth 4-4-4-12 over 5-5-5-12? I realize overclocking increases this latency, but how much farther will CS4 let me push an E6600 with air cooling (say a Tuniq Tower 120)?
Finally, where in the specs can you find out whether a module uses Micron chips? I can't find this info.