Left 4 Dead is clearly limited by the Athlon X2 3800+ CPU--look at the difference 600 MHz makes. Our Athlon 64 X2 4200+ at Athlon 64 X2 5000+ speeds is doubling the frame rate. The game still appears to be CPU-limited with the performance topping out similarly across all of the graphics cards, but at 60 frames per second (FPS), the CPU bottleneck isn't having a detrimental effect on playability.
With anti-aliasing (AA) and anisotropic filtering (AF) enabled, performance is still multiplied, except, strangely enough, for the AGP Radeon HD 4650. We can't easily blame this phenomenon on the AGP bus, because the AGP Radeon HD 3850 is still outperforming the PCI Express (PCIe) Radeon HD 4650. Perhaps the AGP Radeon HD 4650's beta driver is at fault here.