How does one determine wattage ?
If I had a Pentium dual core 2.8 ghz
Two 500 gb HD's
A Radeon 5xxx class video card
The manufacturers usually publish specifications on how much energy their devices will use. You'll want to know how many watts each device uses under full load. Add up all the wattages and you have the total. When choosing the power supply, you should choose one with a higher output than the total wattage to compensate for under reporting wattages used on all your devices, as well as under reporting the actual capabilities of the power supply.
Ruel24 is exactly right because he's considering output power,
but he's looking at in one way. Just to illustrate, I'll use the numbers he gave as an example.
For output (and more generally, DC) power, Watt's Law
states: P = I*E where P stands for power, I stands for current (intensity), and E stands for voltage (electromotive force), so if you have a 12vdc supply rail with 52 amps running through it, the output power is 12*52 or 624 Watts. Hmm... I wonder what the other rails of that supply can deliver? Either this Corsair HX650 is just beastly
or the other rails are a bit anemic (without looking at the numbers I woudn't know).
These numbers will be generally correct for any modern switching supply, as ripple (AC component) riding on the DC rails is usually minimal. (Of course, the cheaper you go, the less clean the output, generally.)
It is important to look at whether these numbers are for sustained or peak output, too. This is where PC Power & Cooling really shines (but all things for a price!).
For AC current (input power), the equations get a bit more interesting, as the alternations in direction of the current flow give rise to reactive losses usually accounted for by a Power Factor which accounts for the total inductance/capacitance of the circuitry. A power supply rated for 650W input power will almost certainly deliver a good deal less to the loads because some is lost in conversion.
DJohnston makes good points about how to budget for a system during build: yeah, look up the specs for everything in the system and budget a little extra for upgrades (like extra drives you might want to add down the road) is a good way to go. I usually build for 20% to 25% more than present need for desktops, and a bit more for workstations and servers (since they tend to have longer life-cycles and get upgraded more often). With high-quality power supplies this is a good rule. Any more, though, and you waste energy.
Well, you've done it again... wasted another hour with Hack and Slash, the Circuit Boyz.