yup thats entirely true, but most of us going for so-called 'duff cpu's' are doing so with the knowledge of the q6600 prices around the corner (i was actually going to get a s775 celeron but figured id like to use the chip in a second build).
but even if you run the e4400 at 3-3.5ghz with good air cooling (xp90C), it still helps to run the ram at >800mhz (may be a little bit, but thats what overclockers do, lots of input for not a lot of gain, anyone can run a e4400 at 3ghz).
I personally always think of how likely it is that the thing will be replaced - i dont envisage moving to ddr3 anytime soon, so a good set of ddr2 will be able to give good performance for some years (im talking 3-4 years)yet.
its an investment, not a stopgap (like the chip).