Source: Left 4 Dead System RequirementsProcessor: Intel Pentium 4 3.0 Ghz /
Display Card!: DirectX 9 compatible 256MB video card /
Memory: 1024MB
Free Disk Space: 7.5GB
Operating System: Microsoft Windows 2000 / XP / Vista
See a snapshot of the requirements as listed on the Left 4 Dead box art here.
The image linked lists the processor requirements as: "3.0GHz P4, Dual Core 2.0 or AMD64X2."
I don't know if my Intel Core Duo T2300 will be up to the task.
/cry
I guess I'll wait for the L4D demo, try it out, and find out.