I'm in similar boat. It's not about needing the 4 vs 2 cores now but say 2 years down the line.
FWIW my reading had suggested that under normal usage (i.e. not playing games while encoding a bunch of videos) the E8400 is faster (stock speeds) than the Q6600 at just about anything, since not that much can take good advantage of 2 cores, never mind 4. Further, the E8400 overclocks better. For now, the E8400 is the obvious choice for most people.
But, whether that's true when you're halfway through the period until your next upgrade, that's the trouble. The Q6600 isnt all that much slower and if your uses do become good with multicore the Q6600 should beat the crap out of the E8400.