You have to keep some things in mind:
1) You need new expensive base parts like mobo (around 200 €) and DDR3-RAM (50% more expensive than DDR2).
2) The perfomance increase really, really depends on the software you use. With no optimization a cheap Core 2 Duo can outplay even the biggest i7 in most games. Nevertheless the perfomance of most games depend mainly on the gfxcard and not the CPU. But don't let you fool by synthetic benchmarks.
3) The i7 needs a lot more power so the performance/Watt-ratio got worse. This means more power, more money, more heat, more noise (or better cooling). The chart you linked is quite shitty as it only compares the i7 to C2D Extreme.
consider the cheapest i7 retails around £260 and matches/beats the QX9770 that retails around £1000!!!
That's not that good comparison... Intel's Extreme CPUs were/are waaaaay overprized. They cost 1000$ in the US... £1000 in your country and 1000€ hereover...
I really recommend to read some good articles/reviews, but I only have some reviews in German right now.