It is not a good practice to use a high-end graphics card like RTX 3070 with less than 600-watt PSU because it can have pretty drastic results. When the system runs at full load, you might suffer from system restarts without warnings, indicating that the power supply cannot provide enough juice to the components. In the worst-case scenario, any of the PC components can die during such failures, including your processor, graphics card, motherboard, RAM, or of course, your power supply.