4550 is pretty frugal, Xbitlabs measured 18.5w while running 3DMark06. Also keep in mind they measure DC power consumption (through the PCI-E slot and external power connector(s) if applicable), so this should be pretty accurate. 3DMark06 isn't as demanding as stress test utilities like Furmark, if you were to run that the power consumption would probably jump to 25w or so. But it's a good representation of the kind of load actual games will put on the card, so while gaming you can expect it to use that much power.
http://www.xbitlabs.com/articles/video/display/ati-radeon-hd4550_4.html
With a bit of Googling I'm sure you can find sites that have measured power consumption of those CPUs to get an idea of how much they use. Same for most of the other components (motherboard might be hard to figure out, you can probably assume 20w or so for that as a worst-case scenario, but it should be relatively easy to find typical power consumption for a 2.5" disk, a stick or two of RAM, etc.).
Also, you can run a surprising amount off a PicoPSU if you're careful about the components you select. This SPCR user is running an Athlon II X4 605e (45w quad core) and HD5750 (keep in mind this is the fanless PowerColor version, which I believe has an undervolted GPU to reduce power consumption and heat -- don't assume any 5750 will work). A pretty potent gaming system for how little power it uses.
http://www.silentpcreview.com/forums/viewtopic.php?t=57219
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