You can take a screwdriver and pry the latch back down into the top of the case. On some of the workstations I've had to cut the side of the latch slot back with a hacksaw blade, but in the Opti I've never had to do that. There is also a tab that slides into the top of the PSU that needs to be folded in.
The problem with dell PSUs is that the cables are made just for the system they were installed in. The bigger ones are modular but it will depend on what harness you get with it whether the cables will reach wher you need them to. Also Dell would use proprietry connections for Dual CPUs, FBDIMM memory, water cooling ETC. They have multi rail designs, so the power from unused connectors won't be available to power other devices. In other words the 850 PSU might be cabled to power dual CPU, and not 8 pin PCIe. There aren't enough slots and bays in an Optiplex to ever require an 850W PSU anyway. A good 600W PSU will power wnything you can connect to that MB. There's a lot more to the question than will it fit.