General Response (without looking at your specific answer)
I went to Meta Stack Exchange (the Meta for the whole SE network) and dug up a question that linked to a Help Center article about your specific topic. If you just want the official policy, read the help center. If you want the community's broader take overall on plagiarism, look at the Meta question, or search https://meta.stackexchange.com for "plagiarism" for other related questions.
My understanding is that Super User's take on plagiarism is exactly the same as the network-wide or Stack Overflow policy. Indeed, here's the same page on the Super User Help Center.
Plagiarism - posting the work of others with no indication that it is not your own - is frowned on by our community, and may result in your answer being down-voted or deleted.
If there is strong evidence in the edit history of both answers of his or her content being modified to match yours with significant quotation or paraphrasing, that is plagiarism.
On the other hand, if your answer simply inspired the other user to change their mind and improve their own answer (without meaningfully plagiarizing you), that's probably acceptable.
Getting the right idea from someone else, and then doing your own research and putting in an answer in your own words isn't plagarism. If your answer is more comprehensive and useful in the long term, it will tend to get more upvotes.
On the other hand, if the other user put more effort into his answer and it looks more complete and well-written, they deserve the reputation they get from it.
One of the long enduring problems on the Stack Exchange network is the Fastest Gun in the West Problem, where often the first person to provide a (correct) answer gets the most upvotes, even if their answer is terse, lean on explanation, or mostly quotes external sources verbatim.
A valid response to the FGITW problem is to garner more upvotes/bounties by taking a conceptually correct but terse answer and re-writing it to have better exposition, more background explanation to set up the reader's understanding, or working out the solution in greater detail or with better pictures or formatting.
If that's all they did, then it's fair game.
If there's enough of your ideas in his post to be plagiarism, it's at least worthy of the community's downvotes, if not deletion.
Specific Response (looking at your answer)
- Appleoddity's answer was the first one to be posted to the question. Given the question's high popularity (lots of views and interest in the subject matter), it probably got a large portion of its upvotes very soon after being posted; the actually correct answers didn't start to pop up until around 18-20 hours after the question had the majority of its views and upvotes already passed over it.
- Agent_L's answer was at 15:57 on September 12, 2017 and got most of it right, at least compared to Appleoddity's original answer. "This is neither ATX nor AT power supply." etc. Your answer came 1 hour, 50 minutes later, and so it could be considered that you benefited from Agent_L's answer too. Unfortunately his answer only has 2 upvotes as of this writing. Ouch!
- Your answer is the most specific and accurate, but came a bit later than other answers that were partially correct. It seems like arriving at the correct answer here was genuinely a team effort by the community, and rather than editing an existing answer, competing answers were posted, each later answer benefiting from the research done by the previous (even the incorrect research, which can be helpful in eliminating some possibilities when searching for the right answer).
- Your answer, in a sense, could be construed as plagiarizing Agent_L's answer. You said "It is neither an AT or an ATX power supply." and Agent_L said "This is neither ATX nor AT power supply." nearly 2 hours earlier. Now I don't think that is a statement that can be phrased differently, so I'm not accusing you of plagiarizing, but in the same sense, the edited answer of Appleoddity doesn't plagiarize yours by stating that it's a Compaq 22xx series power supply.
- Appleoddity's answer is, in my opinion, more comprehensive and explanatory than yours. Besides that, he kept his original incorrect answer intact for posterity's sake to show that he was originally wrong; this is good intellectual honesty.
- The last part of your answer where you provide links to purchase the item is not really a great fit for Super User. That sort of information (providing specifics on "where to buy something") is off-topic for the site, and could possibly be perceived as shilling for those companies (even if it isn't). I would advise you to delete the links.