AnandTech has published a valuable article called "Unreal Tournament 3 CPU & High End GPU Analysis: Next-Gen Gaming Explored". Here's a taster:

We don't often look at single-core performance given how cheap dual-core CPUs are today, but it's important to look at where we've come from over the past couple of years. One to two cores gives us an impressive 60% increase in performance on average, if we look back at our first dual-core processor review none of our gaming tests showed any performance increase from one to two cores. From 0 - 60% in two years isn't bad at all. The performance improvement from 2 to 4 cores isn't anywhere near as impressive, but still reasonable. In our first two tests we see a 9% increase and the third one gives us a 20% boost, for an average 13% jump in performance. If 3D games follow the same trend that we've seen over the past two years, it'll be another two years from now before we really see significant performance increases from quad-core processors. If in 2009 we hardly bother with dual-core chips because quad-core is so prevalent, you'll not hear any complaining from us.

Quad-core gaming is still years away from being relevant (much less a requirement), but the industry has come a tremendous distance in an honestly very short period of time. We're more likely to have multi-threaded games these days than 64-bit versions of those titles, mostly thanks to the multi-core architecture in both the Xbox 360 and PlayStation 3. Like it or not, but much of PC gaming development is being driven by consoles, the numbers are simply higher on that side of the fence (even though the games themselves look better on this side).