DDR RAM comes in higher frequences than PC3200 (400 MHz) for overclocking, plain and simple. No processor on the market has a higher front side bus than 200 MHz so the highest RAM the average user will ever need is PC3200. PC3200 RAM is designed for 200 MHz FSB systems because the actual clock speed of PC3200 RAM is 200 MHz before you calculate in the multiplier (2). 2x200 MHz = 400 MHz. I could go way further in depth here, but there's not a reason to as of now.
Any RAM you see past PC3200 is made for overclocking, like I said. You want RAM with a higher frequency so it correlates with the FSB of your CPU. If you want to overclock to 217 MHz FSB, you would buy PC3500 (433 MHz) ram in theory. Many motherboard do not support ram speeds past PC3200, but some do (mainly mobos that are known for overclocking). Some Intel chipsets allow ram faster than PC3200 as well.
I admittedly don't know much about DDR2 memory. It does start at PC3200 and goes higher and I know that some Intel chipsets <I>require</I> it. It is indeed faster than DDR memory but I'm not really sure how considering the fastest of today's processors are still running at a base FSB of 200 MHz (before the multiplier). You <B>do</B> have to have a special motherboard to run DDR2 memory. Again, the DDR memory that is above PC3200 levels is for overclocking or possibly future processors that will breach the 200 MHz FSB limit. DDR2 is simply a newer DDR memory that goes way higher with theoretical better performances (which still won't happen until processors go past the 200 MHz limit).
If you are gonna overclock, good luck. Overclocking is 50% luck and 50% skill. The luck comes from whether or not you receive PC parts that are up to the task, and the skill comes from knowing how to overclock and not fry anything. I'd personally buy regular RAM and spend the rest on a faster processor rather than spending more money on OC'ing RAM and a slower processor. Today's games care about your videocard and RAM waaay more than your processor. Having a fast processor isn't necessary for any operating system and surely isn't necessary for gaming (when I say fast, I mean like faster than an Athlon 64 3200+ or a P4 3.2 GHz).
If you're hardset on overclocking, Corsair XMS is a great choice. Be prepared to pay literally 200% of DDR ram's price for it, however. OC'ing will not work period unless your motherboard allows multiplier and FSB changes as well as voltage changes. You also need to do research on if your processor is unlocked (open to FSB and multiplier changes). I'd buy a 1 GB stick of PC3200 Corsair XMS. You could buy 512x2, but dual channel is extremely overrated and some motherboards simply will not OC with two ram modules instead of one.
I can't help ya with rambus ram...I didn't get into the PC hardware market until DDR had just came out.
-Lasereth