underclocking vs overclocking?

That all depends on how much you're willing to pay.

maybe you underclock it so the bus rates match up...?

...or so it lasts longer? wouldn't that theoretically make components survive longer?

meh, i don't know. although the original poster said "from a performance point of view," implying that he/she cares not about price, i'd say that oc'ing would be better price/performance. although i too am a little confused. wait till the experts hit this thread.

oooooh, right. voltage is what gets ya. i need to remember that

and i assume there isn't much difference with ddr1, which the OP was asking about also? would the "safe" voltage be lower, probably?

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).