Hey,
 
I know that y'all most (9 year CPU and Mother Problem Incoming) likely can't help but recently upgraded my GPU from a Radeon 7850 to a Rx 480. I am getting a constant boot loop where the motherboard will post but not recognising the GPU I believe and restart. 
 
Post= Gives me the Screen with my processor info and frequency and more and then restarts and option to go to the bios or boot menu and then restarts.
 
I have flashed the Motherboard bios with the 2009 version (latest) and have confirmed the RX 480 to be good, as I threw it in a friends rig and it booted and works perfectly fine.
So to my question , is there a problem with compatibility as the Rx480 has been shown not to boot with older hardware? or is it my user error that it is not working? I know my computer is incredibly dated (9 years on the CPU and Mobo) but it runs the games I want to play currently alright 50-60 fps. I was just looking for a decent FPS increase with a new card as with updates things are becoming a little stuttery. 
 
Computer Specs:
Q6600 @3.2 Ghz
Evga nForce 790i Ultra SLI Motherboard
Radeon 7850 ( Boots with no problem)/ (Rx 480 refuses get past the motherboard posting)
16 GB G.Skill 1333 Mhz ram
256GB SSD
550w Powercolor Psu
 
Any and all help is greatly appreciated as I would love to not have to spend any more money upgrading currently.  My upgrades are slow and steady with purpose so a new CPU and Motherboard will have to really be needed. Hell I am even willing to sell the RX 480 for a lost and pick up a Gtx 1050/1050ti at a very small loss if those will boot properly with my board. 
Again, thank you in advance and I hope to here from you soon!

Is the system restarting due to a BSoD, or not getting that far? If so I'd suggest you use the 7850 to boot the system, then disable the automatic restart so you can see what the error was when trying the rx480.
 
This sounds like the UEFI / legacy BIOS issue yet again... the GPU needs to support a legacy BIOS because the motherboard is too old to support UEFI GPUs. Based on some googling it does not appear nForce chipsets offer a legacy/UEFI setting in their BIOS for graphics cards but please look for a setting in your BIOS just to be sure. Otherwise you would need to find a GPU that supports a legacy BIOS, they are most commonly advertised as having a "hybrid" BIOS. I'd suggest you ask EVGA (or whomever made your RX480) for a list of cards that are still compatible.  

:) this sounds like a good board as I've read some crazy threads about people modding the socket or gpu to use 771 socket xeon cpus and runing them at crazy 4.6+ghz with 8gb ram and newer gpus like 980ti and 1080s.. Prob could use a 1080ti. Haha

http://www.overclock.net/...-lga771-xeon-cpus.html

I got one back from my nephew a couple of months back.. traded a more modern laptop to game on as I thought the old system couldn't play any modern games since it only had 8800 sli in it.. had no idea!

Will be fun to tinker with.. have water cooling for it and all.

I'd say try a different psu..maybe it's not up to snuff anymore.. and look for modded bios for the board.