Upgrade PSU from 460W to 850W in Precision 3630

Alright; thanks for the update; glad it worked out!!

Will this PSU work in a Dell 3640 tower too?

Precision 3640 Setup and Specifications Guide 

Yes, but a couple things may need checking.  Your current PSU isn't mentioned.  So, as mentioned before in this thread, make sure the 3.3v/5v rails are met or exceeded.  Aftermarket PSU's often boast more total power and on the 12v rail(s), but often skimp on the 3.3v/5v rails as compared to Dell PSU and then it doesn't work.

I couldn't pull up an image of your MB.  (I still might try again.)  So I don't know what connection it has.  If it's less than 24-pin, I'm sure there's an adapter for it.

3630 still has 24-pin connection.

Tried again for 3640 MB image - no luck.

@TheCropolite  What brand rtx 3070 did you get in the end? I want to order one but not sure if all brands are the same size and would all fit in the precision 3630.

I ordered the 850 psu thanks to this great thread!

Search eBay for the following: Dell Precision 3630 Alienware Aurora R6 R7 R8 R9 850W Power Supply Part# 9XG5C, HU850EF, 48Y6D.  I bought one for my Precision 3630 since I upgraded to a i9-9900K CPU and it fit perfectly and works fine (if your going to upgrade your 65W CPU to a 95W version like the i7-8700K, i9-9700K or i9-9900K make sure you have the 95W OEM CPU heatsink w/fan part# T57JF KTDJC and the VRM heatsink part# 639V8). 95W CPUs draw more power and produce more heat). 

I have tried 2 Corsiar PSU's CX750 & RM850x neither works...  all that happens is the cpu fan spins a little then stops.... should I try a Corsiar HX1000?

What GPU are you attempting to power?

Depending on your GPU,  not sure that a Corsair HX1000 would help.  May need to try a different brand, like starting with EVGA.  I don't know that I've ever seen a post whereas an EVGA didn't work with a Dell.