Is a gaming machine better for software development?
NO.
CPU For software development, you need lots of cores. For gaming, you need fast but not necessarily many cores. This is slowly changing as newer games are being written to take advantage of multicore CPUs, but the general case is that most gaming machines focus on raw CPU power. For example, in my case, I'm an RoR developer, and during development I run: my editor, mongrel, solr, postgresql, and memcached. Most of the time I also have an open browser, a PDF editor, and iTunes.
RAM Most games will be OK with 2-3GB of RAM. For software development, especially web development - if you will be running multiple servers - you'll want at least 4GB, or even 8GB of RAM.
GPU Top-of-the-line graphics cards for gaming can cost $500 or more. For software development, you can get away with the cheapest GPU you can get. The only aspect of the video card you'll want to concern yourself with is the capability to handle multiple large monitors.
It will actually be helpful if your development machine is so crippled (gaming-wise) that you can't play the games you like to play on that machine. No distractions! :)