I believe it was because the company that produced it charged disgustingly high license fees or something, to try to control the market. RAM manufacturers essentially said, “Fuck you,” and flocked to the DDR standard, which, while technically inferior, was made a hell of a lot more affordable by the mass production.
As for the OP, here’s the skinny… DDR-Ram comes in various speeds, from PC-2100 to PC-4600… however, PC-3200 (also known as DDR-400, or some other variation with that number) kinda became the de facto standard (the reason has to do with matching frontside bus speeds with the RAM frequency), and higher-frequency RAM has generally been in the realm of overclockers. So if you want to upgrade to a DDR system, DDR-400 is the way to go (unless you overclock).
If you’re going DDR2, well, I’d say wait, because it’s still just being introduced, and DDR2 isn’t necessarily faster, it just scales higher… that is, DDR clocked at 200 mhz would be about as fast as DDR2 clocked at 200 mhz. However, DDR2 can reach speeds of 667 mhz and beyond, easily, which DDR cannot do.
Right now, DDR2 speeds aren’t terribly impressive compared to their theoretical upper limit, and plus, it’s expensive as hell.
“Dual channel” refers to a technical tweak that takes advantage of a quirk of the Frontside Bus… basically, with two RAM sticks in a dual channel-equipped motherboard, you can send a greater amount of information at any given moment. Ergo, two 256 megabyte sticks would outperform a single 512 megabyte stick, even though they have the same capacity.
“Registered” memory is something that, in my experience, comes into play when you start dealing with workstations and servers. An Opteron processing requires registered memory, for instance. Since I have little experience (read: none) working on workstations or servers, I can’t give you more info. I haven’t come across a Builder’s Guide yet that recommended registered over unregistered DDR DIMMs, but those are all for building personal, home computers, and I don’t know what your requirements are.
As for your upgrades, I’d suggest a whole new chipset… heck, I’d actually recommend you switch to an Athlon64. Best processor you can buy, in my opinion.