Left 4 Dead 2
Just like Dragon Age, Left 4 Dead 2 employs a DirectX 9 3D engine, and it utilizes all available cores. A single-core CPU is not enough, two cores increase frame rates, four cores show more gains, and even overclocking reveals significant benefits. This game is obviously easy on graphics hardware and more CPU-intensive.
To get more performance, you might want to upgrade the processor and graphics card, starting with the former to take advantage of the multi-core-optimized game engine.
Technically, frame rates are high enough that a single-core CPU is enough to enjoy the game. With anti-aliasing activated, the title uses 760 MB of graphics memory, so you might see some improvements with a 1 GB card.