Well that depends, my HP laptop which has an INTEGRATED graphics chip (HD Radeon 3200 using shared ram off the MB.) can run l4d just fine without lag or problems, but at VERY LOW graphics settings.
IF we however chuck it over onto my main computer (Core 2 Quad q6600, 4 gigs of DDR3 ram on my 790i FTW motherboard, with two pipping 9800GX2s pumping in the graphics flow.) you're looking at a whole new game, full of vivid colors and real crazy looking zombies. And a far better contrast ratio of light to dark.
In my humble opinion this is however a game which the graphics have a HUGE impact on the game play, the eerie atmosphere isn't so eerie when the whole screen is pale because the graphics card cant run with the full contrast going. And the zombies don't look nearly as good without their skin glistening and shinning in the moonlight. They also don't look nearly as nice when they explode. Therefore I have to vote if you want a SICK gaming experience, go for the GUSTO and pick up a new card to run that sucker on max. But if you just want a mediocre experience stick to what you have.
