http://imgur.com/a/x65XmUt
Computer is open. If anyone spots anything glaring I should fix, feel free to lemme know.
Tried the ed2 tip. Got this when I started up.
http://imgur.com/a/Fb7GdYG
Edit: I’ve reached the conclusion that basically the adapter cable is necessary to get the computer turned on. I’ve tried multiple combinations of plugging in the power supply with and without the adapter cable. So any solution would likely have to involve having that cable plugged in to both areas.
Basically means that something, somewhere doesn’t support the lowest power option it’s looking for. It’s a warning only, not a problem, and all it means is that your system isn’t quite as power efficient as the software could make it.
That error is, unfortunately, what I expected. Was worth a shot just in case.
That rules out my crossed-rail theory. I was under the impression the two 6-pin connectors were separate runs, but that’s just a loopback going from one to the other.
Is it at all possible for you to get a photo of that white sticker on the side of the power supply? I don’t want to make any judgement calls based on something I can barely read, but what I think I can make out might have an explanation.
Just theory at the moment, but it looks like that might be a 400w PSU (the “33A-???-400” on the left by the QR code). I was under the impression the XPS 8700 came with either a 480w or 525w based on what I’d been able to find out. 400w “white box” PSUs are frequently overstated in their sustained output rating, and at 6 years (or more) old it could easily be struggling with a GPU that can draw up to 225w (despite its 125w TDP) on its own without exceeding its design requirements.
I can, will get back to you in a few minutes there.
Here we go. Hope this helps.
http://imgur.com/a/WkgwJNY
Cheers.
OK, so I misread that. It’s “33A-0093-A00” - so no indication of power. But it is a 460w unit, which is going to be somewhere between 5 and 8 years old based on the vintage of the CPU. A good quality 460w would be enough to run the system, but I would question the quality of this one and its age will definitely be working against it.
So would a new power supply help? There’s a better-than-even chance, in my opinion, that it would. There’s a fellow who seems to spring up a lot on the Dell forums insisting that whatever replacement you get needs to be able to provide at least 20A on the 3.3v line and a minimum total of 150w on the 3.3v and 5v combined, but I have no idea how he’s reaching that conclusion.
Regardless, that supply you linked earlier - even as a 650w unit - would be more than adequate. They’re a CWT-based unit, which wouldn’t be my first choice, but their gold rated models (which that is) seem to get reasonably good feedback.
However, I should emphasise that I’m still not 100% certain it would solve the problem. It will fit, it will work, but as to whether it will work any better…?
Installation should be fairly straight forward. Unplug the existing PSU completely, unbolt it, and remove it. Being modular, you can attach the cables to the computer (tip: If it fits, it’s meant to go there) before you attach them to the new PSU, which will save you some headaches. Then just mount it in the place the old PSU came from, bolt it in, and away you go. Hang on to the old PSU for a little bit, just in case!
The main difference between what it, or any other modern PSU provides and what you have is going to be the ATX12V connector - the 4-pin plug by the CPU fan with the black/grey/brown wires. More recent PSUs have an EPS12V connector, which is usually provided as a pair of plugs in a 4+4 configuration - one of which corresponds exactly to the ATX12V, and is used in its place, while its “twin” just hangs loose.
It will also offer composite PEG6/8 (6-pin and 8-pin) connectors for the graphics card, in a 6+2 configuration. Use all 8 pins and you can do away with the adaptor entirely.
Thank you so much for your help thus far. I think the consensus at this point is that it’s definitely the power supply. Been talking with my guild as well as posting here about the issue.
The adapter cable was definitely needed as the computer won’t even turn on now without it.
Atm I have two plans going forward. A guildy might have a spare power supply laying around that he might be able to give me. But if that doesn’t work out, I have a clear thing to buy that should work out as you describe.
I have this thread bookmarked to return to later when I have the new power supply in hand. Really appreciate your detailed responses. 
Hey shardling,
hope you get it worked out. kind of a bummer as imo your existing power supply should’ve been enough to run what you have in there/what i and others recommended, but as asterchades was saying, it’s possible it is the issue as it is pretty old and stuff happens so it might not have the same juice it once did. good luck though.
Thanks very much. As I said previously, I was very scared going into this. I know next to nothing about computers but I also know that this rig wouldn’t be able to handle WoW going forward. It was already pushing its luck with Legion and BfA, so I took that leap.
I know that it’s going to be worth it in the end, once everything is working, but right now it’s just kind of…bleh. I haven’t been able to game in days, kind of getting the shakes, metaphorically. xD