When I built my computer, I researched everything for quite a while. I do overclock it a bit, not much though.
anyway, I found that the best ram for my mobo, abit ai7, is the 256 mb. sticks of ocz, I can't remember the speed but it can run much faster than my board supports.
speeds are all theoretical as well, they are close to what they are supposed to be, but never exact. Just like hard drives, they are usually a little smaller or larger in size than advertised.
As far as my ram, someone basically tested a lot of brands, and using the OCZ 256 mb sitcks gave the best performance. In the article he did not know why it worked the best, but it did. So I have a gig of ram running in 4 slots. If I upgrade my mobo, not anytime soon, the ram should have the ability to keep up with increased speeds on MOBO's
Also, hard drives are the slowest part of a computer. I haven't seen anything about new forms of hard drives that operate over 15,000 rpm. or that operate like ram, which of couse functions with magnets so memory can't be retained. It would be interesting to see any new developments, if anyone knows of some.