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! :)
I would say some aspects are the same between gaming machines and development machines, like large disks, a lot of memory, etc. So in that respect yes, a gaming machine would fit better than a low end desktop.
On the other hand, gaming machines tend to be tuned towards raw performance instead of robustness. A development machine often does not need a state of the art graphics card, nor does it want a RAID-0 to spead up the disk. If it crashes one disk you lose all your work, so RAID-1 would be much better. Same holds for memory, ECC (or what its called nowadays) is a bit slower but adds robustness.
One gotcha with powerful development machines is that they do not represent the non-functional requirements as to execution environment. If you are not aware of this enough your software will run slow on a "normal" machine because it ran great on your supercomputer :-) One take on this is that development machines should always be a tad slower than the target machines, but this cuts into your development time. A better solution is to have slower machines in the test environment and a few slower machines in the development lab.
Some attributes of gaming machines can help developers, like having a good deal of memory, or a quad core processor (so you can, respectively, run VMs without hassle, and compile faster).
But a fast GPU won't do you much good, so there's no point in spending much money on it. Unless you plan on developing or playing games, of course.
Summing up: if you plan on using the PC for fun, get a reasonable GPU. If you don't, skip it and keep the rest just like you would. You won't regret it.