My advice as an IT Professional and avid gamer is...
Now I'm not going to be pushing HP for any reason other than their customer support is awesome, something Dell's support USED to be. Thus I'm now supporting HP.
Try getting an HP desktop, something with a nice processor (usually the most difficult part to upgrade) and whatever nice features you like on it, BUT don't spring for extra memory or other things YOU can upgrade yourself. You have the experience and ability to be able to upgrade those as you wish later and at a much better price. You'll be covered by HP's great warranty service for at least the first year and you get interesting little bells and whistles you won't be able to get in such a clean package if you build your own. I personally do just that, use HP machines or cases and build on from there. Its overall cheaper for me and I have a brand name behind the machines I sell my clients. For the top-end machines, yes building your own from the ground up is your best bet. But you aren't looking for that. You're looking for a nice machine and stability. Which you'll get by building on top of a solid machine. The idea I gave you may not be the cheapest, but sometimes spending a little just for stability is worth it, especially if you're not completely at ease building one yourself. You won't be saving THAT much by building your own from the ground up and you WILL have to buy an operating system.
Operating systems (OS), by all licenses sold to the general public, die with a machine. You CANNOT use the OS from an older machine if you change the motherboard and processor, as it is now a NEW machine. Legally anyway. You must replace a motherboard or processor with an exact or similar if exact is not available part in case it fails. You also can NOT remove the OS from one machine and install it on another, to say, well its only on ONE. That violates the licensing agreement you agreed to when you originally purchased or installed the OS. Another reason to go with a pre-built. Of course if you are feeling adventurous and want to build your own machine, you could always save the OS money and use an Open Source OS, although you will again be sacrificing stability and compatibility for money. Just my two cents. Feel free to contact me if you have any questions.
Good luck! Have fun!