Well RAM speed isn't really maxed out. After DDR isn't going to get any better there are several possible RAM technologies which just aren't industrially useful or profitable yet.
A big restriction to everyday desktop RAM speeds is the huge amount of work necessery to overhaul the architectures for something completely new. Currently there's only a wall as long as we build it ourselves.
With enough work and a new architecture we could soon have PCs that don't need any RAM anymore due to the fact a harddrive has more than enough speed to deal with everything on it's own. But my guess is that it still will take several years to get there, if we get there sometime at all. Afterall systems aren't just developed for performance but rather for profit. As long as the two collide performance will always draw the short stick.