In case you didn't know, you can play Portal on Linux using Crossover Games.  I gave the demo a try today, and with a couple minor caveats, I can recommend it.

I have a quad core that can dial up to 2.4 GHz, and only 2GB of RAM.  I had to lower Portal's graphics options a bit, and put my CPU in performance mode, but after that, the frame rate was pretty good, and the load time went down till I almost didn't notice it. 

However, I did notice it.  Every time GLaDOS spoke, in particular, there was a skip.  Portal creation wasn't too much of an issue, but the skip was noticeable.  For those of you familiar with the computing power that goes into Portal's engine when you create and look at a portal, don't worry – the sound file load time is worse than the portal creation delay.  The key word here is “noticeable.” 

One further warning – using Crossover Games to run Steam directly made for an ugly install process.  The installation of Steam itself was smooth (despite failing to find Microsoft's XML whatsit-dingy installer) using Crossover's express install process for Steam via download.  But when it came time to hit the Steam store and actually get the file, Steam launched my Linux copy of Firefox, which needed to send a registered URL protocol to the Steam app, which was a no-go because Firefox was on Linux and Steam was in Crossover's Windows bottle..

The solution:  Install Firefox into the Crossover bottle where Steam is installed, then launch Firefox, go to the Steam store and find the game (for example, Portal: First Slice) and install from there.  Firefox will then launch Steam and get on with the job. 

The options I decreased to get better performance were:

• Shadow detail: Low
• Texture Detail: Low
• Color Correction: Disabled
• Water detail:  Simple reflections

I took a quick screencast.  It came out terrible – no sound and choppy video, even at settings lower than what I describe above.  Still, it's proof of the game running!  Rest assured, sound works, and this is not what it looks like in gameplay.  Also, my mouse wasn't being trapped properly because of windowed mode.  Wah wah wah.

Valve's "Portal: The First Slice" demo running on Linux