I honestly don't know how much it applies to desktops, but certainly in other form factors, you can't rely on TDP as an indicator of the size of power supply needed. TDP tells you how much heat the heatsink needs to be able to dissipate. If you exceed the TDP for a few seconds, there's enough thermal mass that you won't exceed the maximum junction temperature of the chip, as long as you keep your average power (on the scale of seconds or minutes) below the TDP. Indeed, many devices like laptops, tablets, and cell phones rely on this for maximum performance; if you load a web page, the CPU will ramp up well above the sustained TDP in order to render the page, relying on the fact that the CPU will be nearly idle or off while you read the page after it has rendered, so the average power will be below the TDP.
Instead, there is a different parameter - sometimes referred to as EDP. This tells you the power that the power supply needs to be able to provide, more on the scale of milliseconds (so a long enough timescale that the capacitors on the board can't keep up with the power demands, but not so long that the TDP starts to limit you). EDP is always higher than the TDP.
As I said, I don't build my own computers though (I just design them ;-)), so I have no idea if that's something that has worked its way into choosing a power supply yet. It doesn't seem like it, from some googling.
Other things that can make a difference are inrush current and power factor. I have a desktop that has about a 950W power supply. Typical power draw is on the order of 150 watts or so. So you'd think that a quality UPS with a 900 VA rating should be plenty - unfortunately, when powering it on or waking it, either the inrush current or the power factor as it charges all the inductors and capacitors in the power supply exceeds the capacity of the UPS about one in every 10 times. In that case, the UPS lights up its "Overload" light and shuts off, which rather negates the whole point of a UPS. Additionally, because turning on a light switch with a lot of CFLs often causes enough disturbance to the power to trip the UPS, and the UPS activating causes it to wake the computer, it happened that turning on a light switch would often cause my computer to be immediately disconnected from power, as it would hit the overload condition.
So I moved the UPS to my TV and network connections, and put my computer back on a surge protector. Everything is much happier now.