https://www.userbenchmark.com/System/Alienware-18/1352

M18x R3 

Couldn't find the model number. Its a notebook so graphic card is not upgradable. You will have to replace entire motherboard. Go to Support | Dell US and enter your service tag . GO to Documentation to find all the configurations this machine is sold with. 

My suggestion : You may not find a new motherboard, most parts out there will be refurbished, Buy a new computer

@Riccardo89,

XPS_Man is correct. There is no M18 R3 listed. Insert the private Service Tag here to get the correct model number. Then post back with only the model number.

@Riccardo89  There is no M18x R3. People mistakenly call the Alienware 18 the M18x R3.

The AW 18 has the 4th gen mobile intel processors and these are the only line of CPU's that will work. The 4930MX or 4940MX are the best options. 

Yes, the GTX 1070 is compatible but there are different versions of the MXM 1070 and some require chassis modification to physically fit in the MXM slot. There are also Quadro cards that work which include cards like the P5000 and also up to the Quadro RTX cards like the RTX 3000. 

You need the unlocked BIOS to run anything higher than the 900M series as you need to run off SG mode which isn't an available option in the standard Dell BIOS.

All the same GPU options above work for the M18x R2 but not with the M18x R1.