Not really a stupid investment for somebody like me sitting on an Athlon XP box...
The cost for a comparable Core 2 setup would be... err... comparable if we're looking at stock performance. Sure, it will cost 10-20% more but the improvement in interesting areas like workstation apps is - for me - quite important.
On the flip side, overclocking - for me - is the art (as such) of taking the cheapest components and getting them to work better than the more expensive ones. It used to be quite easy, but with new sockets, laser-castrated hardware and ridiculous prices, it's a bit more of a challenge.
For the record, I'm considering Core i7... I have a watercooling setup sitting around doing nothing at the moment, so...