I doubt there would be anything in that Dell worth reusing. Post the service tag number and we can look up its configuration to make sure.
If you want a parts recommendation, we would be happy to give it to you right in this thread. Post your budget for just the computer (without peripherals) and what you will be using it for. Also, I assume you are in the US, what state are you in? Do you have a "source" for an operating system? If not, Windows 7 Home Premium is 100 bucks, 7 Pro is $140. You won't find much love for Windows 8 here, especially for a desktop computer. Works great with a touchscreen on a mobile device.
XP is not going to go "dark" - there just won't be any more support.
It's quite dated, but we have a build your own PC article right here:
Build Your Own PC - PCMech