yeah, most of those peripherals (key,mouse,cam) don't make much difference. typically you can say that an internal HDD takes 10W, an external HDD takes 5W or has it's own separate power supply
the main power hogs are the CPU (50W-125W for the most common ones), and graphics cards (85W-250W for the majority). so even an average 450W power supply should be able to handle a single midrange-GPU setup with 1 hard drive, if parts are kept stock (no overclocking).