DDR3 might be lurking out there, but DDR2 is still the most popular option for most people, and for good reason. It's fast, and currently, very inexpensive. Although there are many PC2-6400 kits available, Kingston puts a twist on their's by offering it with tight 3-3-3 timings.
You can read the full review here and discuss it here.
You can read the full review here and discuss it here.