The E6550 has a fairly low multiplier (7X), so to get a good O/C you might have to raise the FSB higher than other processors. But, if we use that same 400 FSB example, the CPU clock speed is 2.8 gig and the memory frequency is 400 MHz, assuming we are using the 1:1 memory ratio. To calculate DDR2 speed, you need to double the memory frequency, so 400 FSB at 1:1 is DDR2-800.
Ok, let's say you do that and you are happy, at first. But, later you get better cooling and more overclocking experience and your best buddy is bragging and laughing that his rig is overclocked to 3 gig which is faster than yours. Now you want to go faster. Just like processors, most good quality RAM will overclock by a good margin, especially if you feed them a little higher voltage. Might also need to relax timings a little too. So you bump up memory voltage to 2.2v and raise vcore a little and discover with your new and improved cooling that you can run 450 FSB. That's 3.15 gig and DDR2-900 RAM speed. Lots of RAM, like for example Crucial Ballistix PC2-6400 or G. Skill PC2-6400 that is CAS4 will easily overclock to those speeds. Or, you can just get RAM rated PC2-8500 (DDR2-1066).
Hope that helps.