That's a CPU bottleneck if I ever saw one. And it's a surprising one too, because I assumed that Left 4 Dead was a little easier on the host processor than Crysis. Admittedly, our benchmark showcases scores of zombies, but that's what this game is all about. At about 30 FPS it's playable, but not very smoothly. Remember, these are average frame rates, so the minimums will dip a lot lower.
Let's add 4x anti-aliasing (AA) and 8x anisotropic filtering (AF) to spice things up:
Those look like the numbers from the previous chart, but they're subtly different. The CPU bottleneck is obviously still the limiting factor here, but look: the full gigabyte of RAM on the AGP Radeon HD 4650 is giving it a leg up against its PCIe counterpart when AA is applied.
The silver lining is that AA and AF are, in essence, "free" on all the new cards with a CPU bottleneck this severe. The old Radeon X700 isn't doing well, though.