Hi and welcome to ExTS
You don't say which motherboard you have.
Your present Video Card is PCIe 2 x 16
You suggested replacement is the same.
It will be compatible.
The PSU, if it is a direct replacement, will be fine.
The fact that it is 850 Watts is not going to cause any extra heat build-up.
The rating is the maximum that it can supply - not the constant output.
The system will draw what it needs.
For example:
An LED typically requires 25mA.
It will need a current limiting resister but the calculation involves only the voltage and current requirements of the LED.
The maximum current a battery can supply does not come into the equation.