BTW, there's a huge chance that the GTX 660 isn't UEFI ready, why it likely needs a BIOS flash (as did my MSI Radeon 7770) to run in UEFI mode. You can switch to CSM to run the GTX 660, but this may mean that you'd have to reinstall in legacy MBR mode. Better to get the BIOS update.
Here's a discussion about the issue, however the GTX 960 is the clear winner of the two & UEFI ready out of the box. I have a collection of GTX 660/670 models in usage in AMD FX systems, plus a GTX 770 4GB GDDR5 Classified in an i5 system. The 960 is a better performer then the GTX 600 series & can do 4K at 60Hz with it's Displayport 1.2 port.
There's also a firmware patch for (most of) of the GTX 900-1000 lineup, which improves Displayport performance, it upgraded my GTX 960 successfully, as well as GTX 1060/1070 cards. Displayport is faster than HDMI. Even the older DP version 1.2, which the GTX 900 series has, is faster than HDMI 2.0. So there must be some benefit to the 900 series cards to install the patch. Once done, it'll show the update is successful, or doesn't apply to the card. No damage is done either way.
I have had a gtx 970ME running on stock PSU for 2 years now w no issues, just to offer another experience for other owners/readers
@speedstep, you may very well be right about disabling Secure Boot for the non-Dell models.😀
Being it was among the first items I disabled once booted up & updated, and have had three retail cards in my XPS 8700 (MSI Radeon 7770 GHz Edition, MSI GTX 960 & EVGA 6GB 1060 SSC, I can't confirm if Secure Boot would had made a difference.
Although just as you posted, there were other Topics (one very long GTX 970 discussion that could fill a book) on this site concerning aftermarket cards & Secure Boot. The only way I'll find out would be to dig it out (have a dozen PC's), enable Secure Boot & cross my fingers. Still may not boot due to a couple of unsigned drivers, one to improve the Realtek audio that hasn't been officially updated since 2017, installed some that were tweaked from that last driver. Also a tweaked Intel Rapid Storage Technology driver for better SATA performance, as newer official ones are incompatible with the Intel Z87/97 chipsets.