Buenas!, review bastante interesante por parte de PC Games Hardware:
Left 4 Dead: GPU and CPU benchmark review
Left 4 Dead: GPU and CPU benchmarks: Conclusion
If you want to play Left 4 dead in 1,280 x 1,024 with maximal details, an X2 5000+ and a Radeon HD 3870 are enough. With less powerful hardware Left 4 Dead is degenerated, depending on one's personal perception, to a washy zombie hunt. With a Geforce 7 the shooter is almost unplayable.
To supply a 22 or even 24 inch monitor you should, besides a very fast Core 2 or Phenom, have a Radeon HD 4850 or Geforce 9800 GTX+. AMD's Radeon HD 4870 or Nvidia's Geforce GTX 280 are powerful enough to smoothly display Left 4 Dead even with 8x MSAA or special Anti Aliasing modes.
Left 4 Dead: GPUs 1280
Left 4 Dead: GPUs 1680
Left 4 Dead: GPUs 1920
Left 4 Dead: CPU benchmark 1280
Left 4 Dead: CPU benchmark 1680
EDIT: cambiada la url directa de las imágenes, ahora pasa por el site. Así que en principio no deberían de dejar de verse.
EDIT2: eliminadas las previews, el site tampoco permite url directas de las previews.