I am not very knowledgeable about hardware, but I imagine even between two equally rated RAM modules, there may be performance differences.
Nope, like a few people already mentioned, the performance of RAM modules is defined (almost purely) by the number in their name. All DDR-667 ram is exactly the same speed. More expensive ram is only more expensive because it can run at higher frequencies (most important for overclocking) and because it can run with lower timings (lower is better, but apple machines have them set high enough for basically any ram to work so you shouldn't worry about it at all).
Simply buy the cheapest SODIMM DDR667 memory you can get, aside from that your only concern should be module capacity and perhaps warranty time. They will all run at identical speeds.
I bought 2x1GB modules for my macbook a couple of months ago for 70 euros (about 100 dollars now, don't know how much the conversion rate was back then, closer to 90 dollars prolly).
FSB has nothing to do with memory bandwidth. Speak to Intel, they provide the 965PM chipsets. Santa Rosa will only run memory up to 667MHz, even if you put 800MHz modules in.
I think cervaro didn't mean it was Apple's fault, he was simply trying to say that upping the FSB doesn't improve performance much if the memory speed isn't increased too. Although modern systems can of course run at such asymmetrical frequencies without problems, performance is usually best if your FSB and memory frequency are identical.
