It completely depends on what you are developing for. Take target system requirements and double them, and use them as your minimum specifications for development machines. This may seem strange, but it is about the fact that I found what I need, at least when developing various projects.
As others have said, the importance of getting good monitors, keyboards, and chairs is underestimated. If you are going to spend a lot of time on this PC, this is very important.
RAM is cheap, and you probably will not have enough. If you are using 32-bit Windows, the maximum size is 4 GB of RAM. If you use another OS that supports more than 4 GB of RAM (for example, Linux or 64-bit Windows), start with a minimum of 8 GB, and if you are working on multimedia projects, you can be ready to upgrade from there.
The best hit for the dollar on processors seems to be a quad-core processor right now, so I would say that a minimum of a quad-core processor (2.4 GHz or so) should be minimal. You may not see much difference going up, but until you reach the dual quad core, which is a big jump in prices.
Find a reliable hard drive or two. Reliability and speed will be more important than size. Personally, I am currently working on a pair of 640-gram western digital discs on all the machines I create.
Drakonite
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