Mushkin's XP2-6400 2 GB is a rock solid memory kit that offers nice overclocking headroom and voltage scaling. It can be configured to run at a wide range of timings, for example 3-3-3-4, 4-4-4-12 or 5-5-5-15. This will make an investment into this memory safe for when you buy a CPU later which runs at a higher FSB, because you can optimize your timings to fix exactly the clock speed you need. If you don't plan any overclocking at all you can set tight CL3 timings and run at DDR2-666 using the 4:5 divider. If your CPU can run insane speeds like 500 FSB, just switch to CL5 timings.
While Mushkin's price may not be the lowest on the market for a DDR2-800 2 GB memory kit, it is sure fair considering the fact that you get Micron D9 chips, good overclocking headroom and a wide variety of timings to run at.