Overclocking is fun, blowing all your cash on memory is not... at least for me.
As for the more than 15% it varies on what you use your comp for significantly. If you do a lot of 3d work say in 3dsmax or similar it's going to give more improvement than say gaming (as games tend to use a lot less RAM).
PC3000 is really pushing the edges of the technology, they can;t go much further with it, it needs to be redesigned. The other major limiting factor is motherboards. It's getting to the point that they can't make motherboards faster (or we'd all have 200MHZ FSB or so) because the interference and crosstalk on the traces is too much for higher frequency. A new memory architecture is quite likely to co-incide with new memory being introduced.