DDR2 will still run with one stick, but to take advantage of "dual channel" It's good to get a matched set, If it doesn't say "dual channel" on the box with two sticks, it might not work in dual channel mode. "dual channel mode" is related to memory bandwithd.. If two matched sticks of ram are used, they can be addressed simultaniously by the memory controller, instead of one at a time. Supposed to "double" your memory bandwidth.. If you buy one now, and another one later, even if it's the same product code, there may be MFG differences, such as the type of memory chips used, or different timings that might not allow it to run in dual channel mode. Dual channel mode boards are flexible enough to adjust the memory speed, as long as the memory size is the same for each stick. The board will apply the timings and mhz of the slower stick to the faster stick, so that it will still opperated in dual channel mode, but there's no garanty that it will work, so it's best to buy a matched "dual channel" kit right off the bat.

On some newer motherboard chipsets there is something for DDR2 called "SLI"~probly for marketing reasons.. It doesn't do anything "performance" wise; The SLI-Memory thing is actually called EPP~ short for "Enhanced Performance Profiles"

SLI or EPP with the right chipset and bios, will allow more of the timing info and voltage settings to be changed, to help acheive a stable overclock.

Using more than two sticks on a board that supports the "sli" memory option will revert the chipset/bios into displaying/allowing changes to fewer of the memory timing options.