I just wanna say that Quad-Cores arent ''needed'' tho, optimizing games to use it IS needed, since you're a really lazy studio if you don't optimize your engine to fully use the CPU of your players (except for peoples who work for free, like the PCSX2 team
) when it can gives huge boosts in FPS and smoother gameplay.
But using a Quad-Cores isnt needed to play those games, I played Lost Planet, Bioshock and currently Crysis Beta, a Dual-Core has been enough for the two first, overclocking it was fine to play Crysis at Medium-High quality.
A quad-core would only be needed to play games like that at ''ZOMG ULTRA MEGA GFX'' quality, otherwise a good dual-core can perfectly do the job and a better gfx card would be more helpful.
As for games being optimized for QC...
Lost planet isnt fully Quad optimized, but it does use quad cores to achieve better FPS.
As for Bioshock/Unreal Tournament 3 :
"Unreal Engine 3 runs two primary threads and a scalable pool of helper threads. The primary threads handle gameplay and rendering, and provide a constant heavy computing load. The helper threads are scalable to many cores, and handle physics updates, streaming, and decompression now, with threading support being added to other systems over time. In this architecture, a 3-core PC would provide measurably more physics performance than a dual-core PC."
But when tested, Bioshock didnt actually get any ''OMGZ'' improvement from using a Quad-Core
And finally, to quote a dev working on GoW PC :
"We're able to scale the thread-structure pretty well. There is a primary thread for the gameplay and a second one for rendering. On systems with more than 2 cores we run additional threads to speed up various calculation-tasks, including physics and data-decompression. So the overall performance benefits greatly from a quad-core processor. Although we haven't looked into the matter yet, I expect an even further performance increase through CPUs with more than 4 cores in future UE-based games."