If I search on NewEgg for "19A" I get power supplies of differing wattages. What is the relationship between wattage and amperage? How do I choose the correct PSU?
Electrically speaking P=VI, so wattage is voltage times current . So for a 12V rail, at 19A, that is 12*19 = 228W on the 12V rail.
So for example this TT PSU
shows it's amperage as "+3.3V@28A, +5V@45A, +12V@26A"
This means that the power is as follows:
3.3V*28A=92.4W on the 3.3V rail
5V*45A=225W on the 5V rail
12V*26A=312W on the 12V rail
That's the basic idea. The math doesn't follow for the addition of power accross different rails because that varies based on the manufacturer, so yeah.
I hope that answers your questions.
Basically, the higher amperage a PSU can delive over each rail, the better. Certain components use one rail or the other, and some use multiple rails. In general, a 350W PSU is good for a lower-end system, and around 480 is about the max you'll need (except maybe duallies or SLI configurations). The most important factor is the quality of the PSU. A name brand (OCZ, Thermaltake, and others) 350W is superior to a cheap-o 420W that often comes with cheaper cases.[/list][/i]