You can call Dell sales to find out if that PSU is available. This forum post has information about the 850W PSU which should be more than enough for any high end GPU. For example the RTX 3090 requires a 750W PSU.

https://www.dell.com/support/manuals/en-us/precision-3640-workstation/precision_3640_tower_setupspecs/set-up-your-computer

They do not offer a larger power supply for this model

https://dl.dell.com/topicspdf/precision-3640-workstation_owners-manual_en-us.pdf

This is old and end of life model

The good news is that specific 3rd party power supply likely works fine.

https://www.bestbuy.com/site/evga-br-series-700w-atx12v-eps12v-80-plus-power-supply-dc-dc-technology-black/6346160.p?skuId=6346160

No idea if 1000W unit fits in case however

Dell Precision 3640 Tower Model D24M Type D24M004

https://www.dell.com/support/manuals/en-us/precision-3640-workstation/precision_3640_tower_setupspecs/power-supply-unit?guid=guid-c09dd33c-29fa-4da8-99f5-9bdf1e6a42ed&lang=en-us

https://www.dell.com/support/manuals/en-us/precision-3640-workstation/precision_3640_tower_sm/working-on-your-computer?guid=guid-7bf19010-aaf7-4a39-894a-646b3e2f0fc2&lang=en-us

Thanks for the replies. I would be ok to install a 3rd party 850w but i thought it would require buying a cheap 3rd party adapter for the motherboard connectors and I wouldn't like that option to be honest, I read of several of these adapters failing and damaging components. Is that not correct, can I just buy a 3rd party eps PSU and install it without dodgy adapters?

Dell now offers the 1000w psu on the 3640 tower when buying the machine.

Specific 3rd party units work fine in the 3640.

Power supply unit

No adapter needed.  EXCEPT for optical drive power

https://www.amazon.com/CRJ-15-Pin-Slimline-Sleeved-Adapter/dp/B07FK5G2TF

https://www.amazon.com/CRJ-4-Pin-Slimline-Sleeved-Adapter/dp/B07L3963NK/

It uses standard EPS12v power supply.

https://www.evga.com/Products/Product.aspx?pn=110-BQ-0850-V1

Would work fine due to having 160W COMBINED POWER for 3.3v/5v rails.

The 1000w unit from evga only has 100W max on these rails.

Dell 875w units have 225W combined on those rails with 32 and 30amps

https://www.amazon.com/CORSAIR-RM1000x-Certified-Modular-Supply/dp/B015YEI7LK

Not sure if the RM1000x physically fits in the case but the electrical spec is fine.  I wont know until April 15th at the earliest because thats when my 3640 arrives that I ordered on March 3rd 2021.

150 mm (W) x 87 mm (H) x 182 mm (D)

https://www.techpowerup.com/review/corsair-rm1000x/2.html

Note 3.3v rails for DELL 875W unit

Same question: The site has the 1000W as an option, but does not allow you to select it, no matter what other configurations are changed.

I was able to call and order, and the order went through a few hours ago, but now when I check my account, it says the item is not available.

So I'm wondering if this is because of the power supply.

Does anyone have a definitive answer on whether this exists or not?

https://www.amazon.com/15-pin-power-6-pin-slimline-adapter/dp/B01CJWRWG6

Another optical drive power cable

or this adapter

StarTech.com 6in Slimline SATA to SATA Adapter with Power - Slim SATA (F) to SATA (M) - Slimline Serial ATA to SATA (SLSATAADAP6)

https://www.amazon.com/StarTech-com-Slimline-SATA-Adapter-Power/dp/B009JXKQ9O/

Ok great, so at least there are options.

Does anyone know why you can't use the configuration tool on the site to add Dell 1000W PSU to the build then?