It depends on the Dell power supply model and the requirements of the mainboard.  In some cases yes -- in most, though - no.

Depends on which Dell PSU you use, and which Motherboard.

Many higher-end motherboards need an 8 PIN CPU power connector, and I'm not sure if Dell has every shipped

I reused one that i extracted from and XPS 8700 in a budget build and it worked fine. I eventually replaced it with a quieter, more efficient PSU.