DDR2 (Double Data Rate 2) memory has been available for quite some time. New computers, even computers released within the last three years or so come equipped with DDR2 memory. An improvement over DDR memory, DDR2 produces faster performance with less energy consumption. With the way that DDR memory products are designed, none is backwards or forward compatible with the other. For example, if you have DDR memory, you will not be able to upgrade using DDR2 or DDR3 memory. You can only upgrade DDR memory with other DDR memory.
As for DDR3, it is essentially doing what DDR2 has done with DDR. DDR3 performs faster, and consumes less power as it works. The past year has seen a steady decline in price with DDR3, and it currently is about on par in price range with DDR2. More specific information in regards to performance and price will be discussed below.

DDR3 uses significantly less power, and can run much faster. It uses 1.5V per chip, which is 0.3V less than DDR2. Additionally, when it comes to its Data rate, DDR2 (depending on its chip) can run at 400-1066 MT/s (megatransfer per second). In regards to its peak transfer rate, it can transfer from 3200-8533 MB/s. DDR3, on the other hand, can run from 800-2133 MT/s and 6400-17066 MB/s. In regards to the I/O bus clock, DDR2 is only available between 200-800 MHz, whereas DDR3 supports from 400-1600 MHz. Essentially, DDR3, when there are improvements from DDR2, can double in processing power. The fact that DDR3 consumes less power in outperforming DDR2 is impressive.

Ultimately, value comes down with what is best available at the time of purchase, and what your computing needs are. If you are building, or shopping for a new computer and want the best available components available, then DDR3 will suit your needs. Most new computers that are equipped with a double-core, triple-core, or quadruple-core processor will most likely come with DDR3 memory. Although it is possible that some models will still come equipped with DDR2 memory.
For desktop computers, a 1GB stick of DDR3 can cost up to $20 or more; a 2GB stick of DDR3 can cost from $25 and up (some sticks cost over $100). Keep in mind that some of the DDR3 RAM below $50 include brand names such as Kingston. So these are no cheap, no-name brands being mentioned here. As for DDR2, the prices are more or less the same. The difference is so minimal, it is not even worth mentioning. Though it should be stated that the DDR3 price drop does have something to do with that. Naturally, desktop RAM will always be less expensive than laptop RAM, and consumers must always keep that in mind.
The better value, due to the recent price drop of DDR3, will definitely have to be DDR3. Do keep in mind that if your current computer supports DDR2 memory, you can only upgrade with DDR2 memory. Same goes with DDR3 and the original DDR memory.