One of the unique advantages that Steam has given Valve over many other developers is that they can poll user's hardware directly and know exactly what their customers are using. Based on this information, Valve is able to predict how hard a game would be on most common configurations, see new hardware get adopted and estimate how long until it becomes "mainstream". Meanwhile, other developers have had to learn the hard way, timing their game releases incorrectly and thus limiting their sales due to harsh hardware requirements.
Valve does share the collected information, or at least some of it, and just released their statistics for last month. Most interesting in this recent report is the growth identified in both hardware and software. In particular the use of single-core CPUs has shrunk dramatically, whereas dual-core and even quad-core CPUs have gained in popularity. Dual-core CPUs now make up nearly half of Steam users, and quad-core has passed 10%.
On the software side, Windows Vista has seen real growth. Perhaps a little too late, with Microsoft already busy working on Windows 7. Also interesting was the split in terms of screen resolution - 25% of Steam players used 1024x768, 37% used 1280x960 and 28% used a widescreen ratio (16:9). Whatever the reason, people seem to have a very sharp divide on what resolutions they prefer (or perhaps their hardware can handle).