Optiplex 790 MT, GTX 970

i would update the BIOS first prior to the install of the card, and determine whether it is a uefi or legacy card and uninstall the old AMD Drivers(I prefer to use DDU). Also be prepared for the worst, the card may or may not be compatible. I have a gtx 1050 but my optiplex 790 MT does not detect it in BIOS

Is there any work-arounds for that? It has the latest A22 BIOS installed on it already. Also, how could I determine if the card is UEFI or Legacy?

https://www.techpowerup.com/vgabios/163796/evga-gtx970-4096-140930

It says the GTX 970 supports uefi. So this i where it gets interesting...people reported that the card woked fine when they were in legacy boot so you may not have to change anything but i would reccomend it to stay on the safe side. If you want to know whether your pc is a legacy boot or eufi search for system info and it will give you your pc Specs, look for BIOS Mode. Now if you you are on legacy mode and you want to switch to UEFI or vise versa you will need to do a complete re installation of windows but like i said before it will be your decision in the end 

Your issue is related to a power supply that is too small.

EVGA 750B1 is minimum required for a 970.

https://www.amazon.com/EVGA-SuperNOVA-Modular-Warranty-110-B1-0750-VR/dp/B00K85X23O/

B2 BQ G2 models from the same vendor DO NOT WORK.

Here is the exact scenario installed in an XPS 8500 and it works fine.

EVGA SUPER NOVA NEX 750B1  P/N: 110-B1-0750-VR

Thanks for your reply. Should I worry about the card being too big? Or are there any case mods I will have to make?

You have to use the short single fan versions of the card,

3 fan 12 INCH cards will not fit.