Power meters are a great diagnostic tool.
Regarding the "right-sizing" you are trying to do with PSU output capacity. Make sure to err on the side of too much rather than too little. Instability due to voltage sagging will cause you nearly endless headaches with weird symptoms coming from all over your system, both hardware and software. Estimating consumption is a fairly simple process. Just add up the numbers for your various chips (CPU, north/south bridge, memory/DIMMs, and other controllers on the motherboard), add-in cards (video, sound, firewire, etc), and devices (hard drives, CD/DVD, etc), and then tack on 25% or so for error margin. More if you think you are going to expand significantly in the future. For most single-socket desktops with a mid-range graphics card 600W should be sufficient. For a high-end dual-socket systems with high-end graphics cards, 850 to 1000 watts would be the way to go.