I just purchased a dell Inspiron 3670

Unfortunately, a GTX 1080 will require a Dell 460w power supply or higher.  The 3670 was shipped with 365w power supply option can not support the GTX 1080.  The other lower wattages PSU are out of the question.

You can review the system specs for other discrete graphics options, a proper PSU will be required.

https://www.dell.com/support/manuals/en-us/inspiron-3670-desktop/inspiron_3670_setupandspecs/video?guid=guid-aae275cf-0dc3-4ccf-b337-fcc6e787dc3d&lang=en-us

@Chino de Oro​ I have a psu and a 24 to 8 pin adapter. I was just wondering if the gpu would fit in the case.

Standard ATX power supply may fit the chassis but not the mounting windows unless PSU exhaust fan is facing downward direction.  You probably can use SFX style power supply with standard mounting bracket to overcome the issue.

However, verify your 24-pin to 8-pin adapter cable to see if it is specifically compatible to Inspiron 3670.  The old adapter cable for 8-pin Dell systems is not a compatible cable.

https://imgur.com/a/M8sTE9h

it was hard fitting this 1080 in here tho 😭🙏

i want to do the same . Can you tell me where you bought the adapter?