XPS 8960, upgrade to RTX 4090, 1000w PSU, more ram

43YPM 1000W Acbel (XPS 8950, Aurora R13/R14, Precision 3660)
R61D8 1000W Liteon (XPS 8950, Aurora R13/R14, Precision 3660)
RD0G0 1000W Delta (XPS 8950, Aurora R13/R14, Precision 3660)
MP23Y 750W, Acbel (XPS 8950, Aurora R13/R14, Precision 3660)
M92DC 750W, Chicony (XPS 8950, Aurora R13/R14, Precision 3660)
M2G8X 750W, LiteOn (XPS 8950, Aurora R13/R14, Precision 3660)
V63M, 1J62C, V1M9K:  1350W (product line not listed) 

What do you gain from the RTX4090 vs the RTX4070 you have now?  You will have to make sure the new card physically fits inside the XPS 8960 case. And you may have to disable Secure Boot before installing any after-market GPU, or the PC may not boot.

Max RAM is 64 GB. So how you upgrade RAM from 32 GB depends on what's installed now, 2x16, 1x32.  You want RAM installed as matched pairs so it can run in dual-channel mode which is faster and more efficient than single-channel mode.

If you have you have 2x16 now, you'd have to remove both and install matched 2x32 GB since there are only two RAM slots. If you have 1x32, you may still have to replace it with a pair of matched 32 GB modules, unless you can buy a matching module from Dell (CAUTION: high prices), because mixing brands of RAM, even of the same specs can cause problems. 

XPS 8960 uses DDR5 RAM at either 4800 MT/s or 5600 MT/s. If your existing is 1x32 GB, 4800 MT/s, you don't want to spend extra for 1x32 GB 5600 MT/s RAM, because the PC will run all RAM at speed of the slowest module, and that assumes the mixed pair is compatible...