The CPU calculates the sound, the physics, and the AI. With a sound card, only the physics and AI are left.
Anandtech released benchmarks of how CPUs perform in Oblivion. Unless you have a X1900XT Crossfire system (2 x X1900XT), you don't see any improvement from a better CPU in GPU-intensive environments like the Oblivion gates.
The only times a CPU makes a difference is when there has to be a lot of AI or Physics calculations. In the "town" benchmark, there are an ungodly amoung of NPC and the CPU has to control all of em. Oblivion has some of the best AI ever, so its hard on the CPU. Thats the only time when the CPU makes a difference.
99% of the time, you're not going to be in a situation that requires that much from the CPU. The job of the CPU is the above tasks, and giving the GPU stuff to do. A Athlon 64 3200+ can currently handle all of that, however, not even a X1900XT can keep up with the amount of tasks a 3200+ can give it. Only when you go with Dual X1900XTs do the GPUs get enough muscle to overpower the CPU.
Hope that cleared things up.