It depends on your core 2 quad if you have lets say a Q9400 or better then you should be fine with a good graphics card and other components. The I3, I5, and I7s are built on a different architecture all together they are faster clock for clock then the core 2s are. The I5s and I7s will perform about 20% better then the high end core 2s do. Most games are more GPU dependent so a good graphics card is more important then the CPU is. So like I said the High End core 2s will be good for the next couple of years but if you are building a new system then it is pointless to build with a core 2.
The difference between core 2 and I7 besides the new architecture they also include hyper threading so it has 4 cores and 8 threads to the operating system it looks like you have 8 cores.
Then read your post and I can't think of a more suitable answer to your question than this article.
Although this doesn't really guarantee the same will apply to future games, this article does show that on the vast majority of single-GPU systems, the CPU is very rarely the bottleneck. As it says in the conclusion, this may differ on multi-GPU systems but a quad-core should be more than enough even then.
If you already have a Core2 Quad installed, you wouldn't see a big enough improvement in gaming performance to justify the price you would pay for an i7 setup. I'd stick where you are at the moment if it were me. You're money might be better spent on a better graphics card or a multi-card setup. You may wish to upgrade your CPU later on (in a year or two) perhaps to a hex-a-core chip etc. Games are only really now starting to be made to utilise four cores on a processor. At the moment, 3 cores seems to be about the sweet spot but obviously this will change as time goes on. Of course, you also have the option of overclocking your C2Q and squeezing a bit more performance out of it. Either way it still has some life in it yet.
On the other hand, if you're building new then it wouldn't be a good idea to go the C2Q route since the LGA775 socket is now dead - there's no new processors coming out for it so you cut off a vital upgrade path there immediately. In this case you may be better off going for an i5 or i7 setup. The i5 750 performs very well in modern games. Of course, the other option would be the AMD route. This may be sensible since you could achieve more than adequate gaming performance and this would leave you with more money to spend on a better graphics solution instead. The Phenom II X4 chips perform very well in gaming.
The main bottleneck is always the graphics card for gaming (to a point).
So at this point in time you'll not notice any major difference between a high Core2Duo and an i5/i7, this will probably start changing soonish though.
Two more points before I forget: Hyperthreading makes no difference at all for games, also if you're making a new system go for an i5/i7, anything else wont be vastly cheaper and you'll regret it in a year or two.