Quad Core processors are good for benchmarking and doing such CPU-dependent activities as video/audio encoding/decoding, or Folding @ Home, and other things similar to that. But for gaming... it's another story. Most games still use a Single Core, a good number will often use a second Core from time to time, either because the GPU drivers off-load some Vertex/Shader computing to the second Core (such as the ForeWare drivers), or simply because the game was programmed to be computed in multiple threads.
Multi-threaded games are more and more popular, and of course are made in greater numbers since the past year or so, but still, today, I haven't met a single game where two Cores were "fully" used, to a point where a third and perhaps a fourth Core would be required (key word) to resume game-play. Most of the so called multi-threaded games will use something like 70% or 80% of one Core, and then around 30% to 40% of the second one. There are a few exceptions such as Supreme Commander where the first Core is used to about 95% and the second to 60% or so, but such games aren't made each months...
With that said however we should, eventually, prepare ourselves for a multi-core requirement era, and we can already see the tip of that iceberg today, but it's still a tip and nothing more at the moment, especially to the eyes of a gamer. Of course all of this is just my two cents on the subject, but there are still facts. No games today and to be released this year, as far as I know require three or four Cores to actually launch and play them.
But just like when Dual Core CPU's were first released, what were most gamers saying? "It's still useless, there's no games using two Cores as of now, it'll be worth it only if you do video encoding and stuff like that, and to increase your 3DMark score". But what was the "better" reason why most people didn't move to Dual Core? Think for a second... done? Because the Dual Core processors were not affordable. It's simple. Today you can hardly find a Single Core, most of them are now Dual Cores and they are even more affordable than some Single Cores only two years ago. It's a new trend, and to some extent a necessity today. They are still doing their job fine and making any systems run perfectly well, but we are now using more than a Single Core in most of today's applications.
It's still a question of adaptation and prices, it hasn't changed much over the years did it?
When the Quad Core CPU's are more affordable people will make the move and slowly but surely leave their Dual Cores behind. The "good" (not the "extreme" ones) Quad Cores today aren't very affordable, and the less expensive ones are always cut in features or capabilities (just take the Q9300 for example, it seems at first glance that it is an affordable Quad Core but then it has half of its counterparts' L2 Cache cut, it has a very low multiplier, and quite a significantly slower base frequency). The day that Quad Cores such as the Q9450 or perhaps the future revisions of it will be as affordable as a fast and efficient Dual Cores of today will be the day where people will suddenly forget all they've said "against" the purchase of a Quad Core and will then praise them instead and minimize the efficiency of Duals.
Market and technology adaptation, consumers adaptation, time and prices. It's always the same story.