DDR Ram transfers data on both side of the clock-cycle, effectively doubling performance. The various numbers refer essentially to the speed.
The type of core logic chipset that is incorporated in the mobo dicates the type of RAM that can be used. PC2100 DDR Ram, which operates at 133 Mgz X 2 = 266 Mgz is the DDR de facto standard right now. Recently, VIA released a chipset which supports 333 Mz DDR, which is a bit faster.
Memory speed is important on higher-end machines as older, slower memory can’t feed data to the processor fast enough, effectively bottlenecking the system and slowing it down. Rambus memory is theoretically the fastest, but it’s high cost led Intel to abandon their Rambus-only architectures. In the real world, DDR seems to offer comparable performance at a lower price.
You’ll see many of the large vendors offering steeply discounted Pentium systems based on PC133 chipsets as they try to clear their older inventory. While they often have fast processors, overall performance drags a bit when compared to a similar system with DDR or Rambus. Depending on the price-cut, you may or may not want to invest in one.
The clock-speed wars between Intel and AMD have effectively brought us processors that in most cases greatly exceed the needs of most users (exceptions - graphics artists, gamers, etc). One more reason why I buy AMD - Intel needs the competition to keep them honest. Anyone remember the first Pentiums?:
60, 66, 75, 90, 100, 120, 133, 150, 166, 200 mz, each at a significant price premium over the previous release for only minor performance gains. Throw in viable competition, and we now have 2+ Gz processors with relatively short life-cycles between iterations.
BTW, IMHO now is a good time to buy a machine - prices are low due to short demand, and performance won’t scale up too quickly until AMD catches up in clock frequency fight (even though their chips generally perform as well as Pentiums rated at much higher speeds).