Underclocking the ram to match the processor and tightening the timings will probably give you better performance and be easier to do with less voltage, but it will cost more. Underclocking is a lot easier then overclocking, and if you're overclocking your FSB and RAM at the same time you have to do extra testing to try and see which one needs more voltage to go faster if you're trying to keep them 1:1. You also become limited by whichever one is slower. With fast ram you could, for example, set the ram to ~800 with relaxed timings and leave it there until you've found the FSB you want to use, then drop the ram down to match it and tighten the timings (1:1 ram at the tightest timings you can get is supposed to give the best performance). I don't have much hands-on experience with the latetest sockets or DDR2 though, so hopefully someone will correct me if I'm wrong.
What chip it is does matter more then how fast the OEM says it will go. Some of the really high-end stuff goes through extra testing and is speed-binned, but you pay a huge premium to have them test it for you. If you do the research and get the right chips you can be fairly sure of how fast they will go and just overclock a more mundane model (that's all the OEM does, they just have fancier equipment to do it with).