Q9400 is a Core 2 Quad on socket 775 - its finish of life and a dead-end. You may acquire a cheaper system now but you will require purchasing new CPU, RAM and motherboard (as a minimum) for the next machine as there will be no promotes.

At least the i7 has some degree of last aptitude in it. When you say the i7 runs at 2.66GHz I suppose you're referring to the i7 920? That runs on socket 1366 motherboards and they do contain some legs in them with the 6-core i9 on its way next year.

The i7 920 is greater to the Q9400 even at the same clock speed, so personally I'd complement a bit of last ability, create the extra investment now and go i7 on socket 1366. You can go i7 on socket 1156 but I'm unsure how much durability 1156 has in it.