If you plan to crunch a lot of mp3's, video or the like, a newer quad could suit your needs better than a C2D. If you don't then I would not lose any sleep over it. Even if you have a sizable music collection to convert, once you do it then it's done. I would only upgrade if you think you are going to crunch a lot of those kinds of files on a regular basis.
As for your question of the C2D getting old, well, technically yes it is getting old. A good comparison would be a CD vs. your C2D; the C2D is the new CD and is not as fast as the new processors.
Considering there is the i5 (socket 1156), i7 (socket 1156) and the i7 (socket 1366) the C2D
is getting long in the tooth but don't let that scare you into upgrading unless you need it.
When I bought my dual processor 2.7 G5 LC it was the fastest I could get. It crunched a LOT of data back then for me but these days I don't really have the need for a MacPro. Unfortunately, for me, there is a huge jump from most of the iMac line to the MacPro. IMO the top iMac is overpriced, for what it is, and is not an option for me.
Hackintosh? Perhaps.
