Dell Precision M6600 Graphics Card: Best Possible Upgrade?

If you can upgrade to the above do so. Make certain there are good Windows 10 drivers for these cards. My experience with Nvidia is that constant OS upgrades is not their strong suit.

I have an Intel HD 520 in my Skylake machine and works very well for me and withstands all OS upgrades.

in the ^specs, they list these :
• AMD FirePro M8900 Mobility Pro with 2GB GDDR5 dedicated memory
• NVIDIA Quadro 3000M with 2GB GDDR5 dedicated memory
• NVIDIA Quadro 4000M with 2GB GDDR5 dedicated memory
• NVIDIA Quadro 5010M with 4GB GDDR5 dedicated memory

this seems to be the fastest :   NVIDIA Quadro 5010M with 4GB GDDR5

You can make your own comparison on the recommendations here:
notebookcheck.net
The architecture of Quadro cards means you actually lose some gaming power as their resources are more targeted at rendering 3d graphic designs than RPGs but they are the better options and a whole league better than the FirePro.
Fundamentally though this big beast is a hybrid Desktop Workstation/Gaming Laptop built for graphics work so there is a limit to the high res FPS you can expect to achieve and even with switchable graphics components it was never going to be a gaming star.

If you do go for the Quatro make sure you get the Dell card - Space in the case under the palm rest for the GPU is very tight and cooling is an issue.  The Dell cards have had to compromise on depth and the normal thermal pads on AMD mobile GPUs are missing and thermal paste use to compromise so that the case will close properly.  If you buy a third party daughter card you may find you need to make those modifications yourself.  The stock AMD cards are at least 0.5 mm too thick with the pads in place.  The Quatro heatsinks aren't an issue.

I think these are probably your options (With Dell P/N's)

nVidia Quadro 1000M 2GB GDDR5 KDWV4

nVIDIA Quadro 3000M GDDR5 2GB 7CD45 RDJT7

nVidia Quadro 4000M GDDR5 2GB Video Card DR11K HGXY3

nVidia Quadro 4GB GDDR5  K4000M Video Card 5DGTT

nVidia Quadro 5010M 4GB Video Graphics Card  K9MWW

AMD FirePro M6100 AMD 2GB GDDR5 Graphic Video Card K5WCN 0MG0X9

AMD FirePro m6000 2GB Video Graphics Card - FHC4H

AMD FirePro M8900 2GB Video Graphics Card - 6W46K

Select allOpen in new window

But the higher end cards are not cheap and you may feel that it would be better spending the top end money on a newer laptop.

HTH

here - second hand unit at a reasonable price:

https://m.ebay.co.uk/itm/Nvidia-Quadro-3000M-2GB-GDDR5-GPU-For-Dell-M6600-M6700-HP-EliteBook-8760w/263445301866?hash=item3d56904a6a:g:NdkAAOSwHYpaRSEf

there should also be a heatsink too:

https://m.ebay.co.uk/itm/NEW-OEM-DELL-Precision-M6600-nVidia-QUADRO-3000M-GPU-Fan-with-Heatsink-07JMFV/112378786183?epid=1434616339&hash=item1a2a4c3187:g:YkQAAOSwbYZXUClf