Left 4 Dead runs surprisingly well on my computer - even better than Team Fortress 2, which still suffers from the same mysterious FPS woes that are unique to its version of the Source engine. There is still something clearly wrong with TF2 on the PC, since I get the same choppy, inconsistent, but still playable performance regardless of how much I raise or lower the graphic settings or the resolution. Every other Source game I own - including Left 4 Dead - runs much smoother than TF2.
Like TF2, I'm running the game in 1440 X 900 with a combination of high, medium, and low graphic settings - though there are a few more mediums and a lot more lows than previous Source engine titles. It's important to note that even the lowest graphic settings don't look bad at all. Usually, if you have an old computer that can barely run a game, the lowest graphic settings of even the most advanced games make you want to quit to desktop, put a burlap bag over your monitor, and forget the whole sorry business of gaming on the PC.
Not so with this game. I need to take some screenshots to compare, but Left 4 Dead's lowest graphic settings look to be about par with the original Half-Life 2's highest settings. Unless something is drastically different with the full release, as long as you can match the minimum system requirements, then you're going to get a pretty game and a solid FPS count. Now that's how you make a game on the PC.
Also, there are some noticeable improvements to the Source engine, including several problems that have been with the engine since it's debut in 2004. The long load times just to boot up the game are gone. On my computer, Left 4 Dead loads to the main menu even faster than Half-Life 2. I went through the demo's two levels three times today; only once did the game stutter - and that's with the sound quality on high. Having a single gig of ram, I've always had to crank down the sound quality to keep Source games stutter free. I forgot to lower the sound quality before playing, and it wasn't until the game stuttered that lone time that I remembered to lower the sound quality.
If the full game runs as well as the demo, I may not even bother with the 360 version. The wife and my wallet will no doubt thank me for that.