Bioshock 2 is also based on the Unreal engine, and it responds to multi-core processors and overclocking. A single-core CPU will slightly bottleneck the graphics card, so, ideally, you want two or four cores to fully shift the burden onto this graphics card. The performance increase between these three different core scenarios is so small, however, that an upgrade is rather pointless.
The graphics memory numbers here are interesting. Bioshock 2 only uses 430 MB with a single-core CPU, but 735 MB with a dual- or quad-core CPU. Whether the 10% performance increase comes from the second core or from the optimized use of resources is hard to say. Most likely, it's a combination.
When it comes to upgrades, consider a CPU with at least two cores or you might waste up to 10% of the graphics card's potential performance--and that's on a GeForce GTX 460. A higher-end GPU will be even more dependent on a capable processor.