In the end, in spite of my original thoughts, and after doing some research, I've decided to go for the E8400 which, given my usage, should give me the best perfomance/price ratio for today's and near future applications and games.
I'm sure more demanding apps and games will come out in the future and will make a better use of quads. No doubt about that. The question is when, how many and am I going to use them? My personal guess is by the time it makes sense to get a quad because most applications are optimized for it, there'll be much better quad cpu's out there than the "old" 65nm Q6600 and "not so new" Q9450, which will only be average by then.
It really depends on what you want:
- Best possible performance now and struggling in 2 yrs time but ok since you'll upgrade the cpu: go for the E8400
- Inferior performance now relative to other cpu at same price (in games), and average perf in 2 yrs time when apps and games use quad threads more, and no intention to upgrade by then: go for the Q6600
I’d like to add that if it wasn’t for the games, I wouldn’t even upgrade my Pentium D 925 3GHz, as it’s doing a great job in all applications, and provides more than enough power to watch films, browse the web, use word and excel, encode music and films every now and then etc. Games are what make the upgrade necessary, as my Pentium was bottlenecking my graphics card (ATI x1950pro). So as long as games benefit more from higher clocks than multiple threads, the E8400 wins.
Quads will be the future, but by the time they are, better cpu’s will be on offer and current quads will be obsolete.