Would it matter if it is an open architecture PC where you buy the OS separately? IDK. I haven't done that for a few years and things might have changed.

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Usually if the computer has been in service 4 years or more, money is better spent on a new computer.

If you do an upgrade of the current steed to a new OS, you may or may not be able to find drivers for things that work perfectly now. Some things that work with xp do not get along with Windows 7 or 8. The list of these is pretty long.

If you’re going to rebuild, and the computer is 3 years old or greater, get a new hard drive. If you don’t use a lot of drive space, you will *love* the performance gain provided by a SSD drive. If you do use a lot of drive space, get a SSD drive for the boot and major programs, and also a spindle drive for inexpensive storage.

3 GB of RAM would make for a cramped Windows 7 machine.

Nope. With a single exception not relevant here (because sixto wouldn't be asking these questions if the exception applied), Win7 requires you to register it, and it does that online through an algorithm of associating hardware with the key. I actually don't know if a hard drive swap alone would trigger it, but in any case when you can buy the software for $60 why bother?

As long as you don't make a habit of it, the algorithm isn't too strict about re-registering once or twice with different hardware.