Lasher, when you know nothing about the topic, simply decide not to offer advice, saves everyone from spending time correcting you..
I could take the time to dissect your statements, but its easier just to say disregard all of the above...
Ket....
The PSU's used in most, if not all retail builds, use almost exclusively, power supplies that are either proprietary or JUST enough power for the configuration they are shipped with...
Your idea of adding a drive bay Add-On PSU is actually a good one...
All it is, is a supplemental PSU, specifically for powering the video card separately from the rest of the computer...
It takes the burden from the Main PSU and allows for stable clean power directly to the video card...
It is linked to the main PSU and turns on when the rest of the computer does, but is independent of the rest of the system in terms of power...
It has its own PCI-E connector coming from it to power the video card, and its own independant power cord going to the outlet...
So more specifically....
A. Yes, you can replace the PSU in that particular Dell with no compatibility issues, however it may be easier to simply add the Bay PSU to power the VC
B. No, the PCI-E cable coming from the current PSU is most likely NOT enough to power the addition of a decent video card...
Its not really the cable that is in question, but the overall power capability of the PSU to handle more additions...
C. Not sure exactly what your asking there...