E6600. My wife can do anything she wants to on an Athlon X2 4200+ and I suspect she'll be able to keep running it for years still.
Anything over 2 cores is wasteful.
If you are concerned about future proofing the i5 would be my other choice.
This is the most foolish thing I think I've read in the past month.
Yes, dual-cores can and will run decently.
But, quads of decent quality definitely show benefits over the duals in lots of situations, notably multitasking, encoding, and gaming. And even if you are only running 2-3 apps, your windows installation is still juggling DOZENS upon DOZENS of threads in the background of varying intensity.
Quad > dual. End of story.
I realize this is only your fifth post here, but
try not to spread obvious FUD with your 6th and later posts.
Oh, and there was no i5 mentioned at all in the OP. If there were, there are dual i5s and quad i5s of that first core i-series gen anyway.