My Acer Nitro N50-650 upgrade questions Can this pc replace with a psu of 650w

No, the N50-650 has an Acer proprietary mainboard and uses a proprietary PSU of either a 300W or a 500W and the mainboard works on ATX_12V-Auxiliary 8-pin power connector and an ATX_POWER1- 6-pin ATX power connector which the new higher modular PSUs are not suitable for the Nitro N50-650 mainboard connectors. The N50-650 uses these below gpu's from Acer:

If you want to fit an RTX3070 gpu that needs 650W PSU, get a new mainboard, as that is the best solution.

N5-650 mainboard and connectors

N50-650 OEM PSUs

The Nitro N50-650 desktop does have a PCIe4x4 slot, see mainboard caption above at #19 M2_2 Slot -as that is an M.2 (2280) slot for PCIe Gen4 / SSD, the below specs are all the drive specs for the N50-650 desktop:

With the second part of your question, why would you put a new and now very expensive Gen 5 NVMe SSDs drive into this desktop? Don’t waste your money as the PCIe4x4 M.2 SSD drives are fast enough and if you put a Gen 5 M.2 SSD into this desktops Gen 4 M.2 slot, IT WILL DO NOTHING AND WILL NOT GIVE YOU FASTER SPEEDS! The Gen 5 M.2 SSDs are backward compatible and will work in a Gen 4 slot, but why would you waste your money? Stick to the PCIe4x4 as they are quick enough for whatever you will do, as the new Gen5 M.2 SSD drives speeds can only be utilized fully if you have a Gen 5 M.2 SSD mainboard.