In computer memory parlance, a rating such as PC-4200 or PC2-4200 is a numerical value that represents the amount of data that a particular piece of system memory can push in 1 second. The only difference between the two terms is the type of memory. PC-xxxx refers to DDR (Double Data Rate) and PC2-xxxx refers to DDR-2. Also, these types of memory can also be described by their clock rate in megahertz (MHz); for example: DDR-400 (MHz).
DDR
DDR, or Double Data Rate, is RAM that performs two operations per clock cycle. The internal speed is half the external speed. There is no official specification for PC-4200. The highest standard that the JEDEC Solid State Technology Association ever approved for DDR was PC-3200, or DDR-400 (running at 200Mhz internally.) However, an individual stick of RAM (Random Access Memory) or memory might be capable of running at a higher speed through overclocking. Using this method, speeds of PC-4200 can be achieved.
DDR-2
DDR-2 is a newer generation of memory technology. It allows for higher clock speeds as well as greater bandwidth. The slowest version of DDR-2 PC2-3200. An interesting fact about DDR-2 is that it actually runs four operations per clock cycle. So while a piece of PC-3200 memory runs internally at 200MHz, a piece of PC2-3200 memory actually runs at 100MHz internally; both have an external bus speed of 400MHz. The upshot of this is that DDR-2 reaches higher external speeds than DDR, at lower internal clock speeds.
Latency
Another fact to consider is latency. Because DDR-2's internal clock cycle is slower than DDR memory, each individual clock cycle takes longer to finish, even though more operations are taking place per clock cycle. This means that when comparing two stick of memory that have the same bandwidth (3200 Mbit/second, for example), the DDR stick will actually perform slightly better. Of course this is offset by the fact that DDR-2 can reach higher overall speeds. This is why DDR has been almost completely abandoned.
Cost
Because DDR is no longer in large scale production, the cost per gigabyte has gone up sharply in recent years. At the same time, because of its increased popularity DDR-2 has reached all-time low costs, as of November 2009. It's also likely that increased adoption of computers has had a lowering effect on DDR-2 prices. Finally, DDR-2 has probably hit its ultimate low as far as cost, because its successor DDR-3 is starting to take over larger segments of the market.
The Bottom Line
Since no one is building motherboards that support DDR anymore, it is impossible to build a computer that is based on it. Even when upgrading the memory on an older computer, it might make more sense to upgrade the memory, motherboard and processor to the DDR-2 (or even DDR-3) standard, both because DDR-2 has a longer future than DDR and because it might actually be cheaper than buying large DDR memory sticks. Also, the large (1 gigabyte) DDR sticks tend to have compatibility issues with most motherboards. It's best to upgrade now, while DDR-2 is at its cheapest.
Tags: clock cycle, actually runs, clock speeds, Data Rate, Double Data, Double Data Rate, external speed