http://topics-cdn.dell.com/pdf/inspiron-3668-desktop_service-manual_en-us.pdf

If you look at the service manual you will note that it has an X16 video slot.

HOWEVER ; Non dell cards will require F2 CMOS Secure Boot OFF and CSM ON and Legacy Option Roms ON. When installing an after-market graphics card into a Windows  PC with UEFI  secure boot enabled, the system may not boot. The Link Below is NVIDIA but ATI has the same Issues as do other NON DELL vendors.  The main issue right now is that most cards are wicked expensive.  There is a specific GTX 1030 thats reasonable and works fine.Gigabyte GV-N1030D4-2GL GeForce GT 1030

https://www.amazon.com/Gigabyte-GV-N1030D4-2GL-GeForce-Computer-Graphics/dp/B07C3DWK1Z/

http://nvidia.custhelp.com/app/answers/detail/a_id/3156/

This user did upgrade the machine with an Gigabyte RX460 so there are a few choices. Also BEWARE there are Tons of FAKE cards out there now.

https://www.youtube.com/watch?v=Ved84d_6occ

GV-RX460WF2OC-2GD

https://www.youtube.com/watch?v=J81h1dHNwDk

https://www.amazon.com/Gigabyte-GV-RX460WF2OC-2GD-RX460-GDDR5-PCIE3-0/dp/B01K1JV83C/

I have added Asus 4gb GTX 1050Ti Graphics card

SanDisk 480gb SSD drive

Also added me spare 300gb 3.5" hard drive

Pc is now super fast with great graphics

all working with the original PSU 240w

You must show me how you did that please tell me what i have to do 

What should I be looking for in terms of physical dimensions for a graphics card, e.g. low profile, maximum length?

I've made the mistake before of buying cards that didn't quite fit, which was pretty frustrating. And I've never owned a PC with such a small form factor before.

I have this card

https://www.asus.com/uk/Graphics-Cards/PH-GTX1050TI-4G/

Try https://www.asus.com/uk/Graphics-Cards/PH-GTX1050TI-4G/

Att.

Márcio Nugas

Is that a good graphics card for gaming because I have the same pc and I want to make it a little better for gaming

I would like to install a gtx 1060 which model can I use I have a i7 7700 and 16gb ram which power supply do I need to upgrade to

I know this is a little late but I was wondering if this was pc was still working even with the 240w psu. (i have the same pc and was wanting to upgrade the gpu)

so i just buy the graphics card that you linked and put it in my computer? that simple or do i have to make some changes because i want to get the same graphics card.

GTX 1060 is not working at all EVER with stock 240W power supply.

The power supply is proprietary.

The 3668 is itty bitty low profile box so any card other than the GT1030 is not likely fitting.

PNY  VCGGT10302PB

https://www.pny.com/geforce-gt-1030

Dell A9763687

https://www.dell.com/en-us/work/shop/accessories/apd/a9763687

Following OEM Dell cards work with upgraded power supply  the Video is for a 3650 the 3668 uses 24 to six pin instead of 24 to 8 pin connector.

https://www.moddiy.com/products/Dell-OptiPlex-3040-3050-3060-Main-Power-24-Pin-to-6-Pin-Adapter-Cable.html

https://www.youtube.com/watch?v=SCQTj1ei3Ew

CARDS BELOW ARE DELL OEM

DVP9W Nvidia GeForce GTX 1050 Ti,  4GB, 75w
36V90 Nvidia GeForce GTX 1060, 3GB, 120w
24K8H Nvidia GeForce GTX 1050,  2GB 75w
8CCF1 Nvidia GeForce GT 1030, 30w
1MPR3 AMD R9-360 2GB, 75w
24K8H Nvidia GTX 1050 2GB, 75w

8MXMJ Nvidia GTX 750 Ti 2GB, 75w
0TC2P0  NVIDIA GTX 745 4GB , 75W
J27RG Nvidia GT 730 2GB,75w

When installing an after-market graphics card into a Windows PC with UEFI secure boot enabled, the system may not boot.  This is true for ANY PC not just Dell.

http://nvidia.custhelp.com/app/answers/detail/a_id/3156

Short Single Fan cards would Require Power supply to be OUTSIDE the case with MODDIY Adapter for the 24 to 6 pin connection.

the hole left by removing the internal power supply.

Less than full size case

Proprietary Power connections

Thank you for the bios tips. I just installed an MSI GeForce GTX 1050 Ti 4gt oc. it is working perfect with the stock 240w psu. All I had to do is adjust the bios settings like you said.

Hey speedstep, I have some questions if you can advise me. I have a inspiron 3668 

But the one got it's very basic 

 and I want to update with a I5-6700

 I just bought 3 monitors I wish to use with graphics card

 are they compatible with the motherboard and what kind of power supply can I update for the Walt ? And any kind of adapter for the wires 

good question. In theory 3668 has H110 chipset so it should support both 6th and 7th gen intel cpu. In practice no evidence that a 6th gen cpu runs in this pc, as Dell only ships it with 7th gen cpu. No user has reported downgrade from 7th to 6th gen in this model. Otoh do not hear anyone say they tried but it did not work either.