Sounds like the cpu is seated incorrectly if you updated the bios before hand or maybe a faulty cpu. Also it wouldn't hurt to make sure all the cables on the power supply are seated properly on the motherboard.

yea i tried reseating the cpu with no fix, i am on bios version A18 if that helps

Take out the new cpu and put the old one back in and if the old cpu it could be that you bought the cpu that was meant for a small form factor when you needed the cpu for a mini tower or vice versa.

If the cpu isn't the right voltage then that could be the issue. 

Many Optiplex MT's, DT's, & SFF's (with the same model # of course) take the same CPU's.  Exceptions are USFF's and micro's.

Try the non-k version of the I7-4790.  The I7-4790 is 84w and the I7-4790k 88w.  So, if the MB limit is 84w, a PSU upgrade won't make a difference in this case.  Besides, Optiplex BIOS doesn't allow for OC anyway.

The I5-4670k is 84w.  I did compare it to the I7-4790.  I see why you tried the I7-4790k.  A 0.2GHz gain is nothing noticeable.

So you think it's the motherboard not giving the CPU enough power? I thought the chipset supported the i7 4790k. 

I believe it's either that or a defective CPU.  In this case, more likely the watt limit.

I don't have documentation on all the CPU's the 7020 can take.  Dell made Tech Guides for the 7010 & 9010 & other models, but not for the 7020 & 9020.  The spec sheet for the 7020 wasn't much help either.

Dell does their own custom implementations.  So the chipset might support more CPU's than what the MB can take.  I guess Dell sells more PC's that way.