Well, probably the most obvious things to look at internally are the capacitors and the transistors/diodes and their heatsinks.
The 2 big capacitors act as an energy storage bank which take energy direct from the mains (remember that mains voltage varies from positive to negative, through zero) so storage is essential for uninterrupted power. It matters little what rating these capacitors are, except under unstable power fluctuations - e.g. if your power goes out for about 0.05 seconds - then large capacitors may be able to tide the PSU over, but with small capacitors, the energy will be exhausted and the PSU output will fade. The critical specification is the capacitance (so 800 uF is better than 100 uF). But there may be differences in the connection circuit - it's possible that 2 100 uF caps may be connected in parallel (giving 200 uF), and the 800 uF caps be conected in series (giving 400 uF). Higher power PSUs need bigger capacitors in order to maintain the PSU output in order to meet the ATX minimum spec of 0.016 seconds at maximum load.
You'll tell little by looking at the transformers - typically there is one big transformer which powers all the power rails, a smaller transformer for the standy power rail, and a 'feedback' transformer which takes control signals from the low-voltage regulator circuit and transmits them to the high-voltage switching circuit. If the PSU is PFC, then there will be a large inductor which may look like a transformer. Size depends on desired efficiency, core material, maximum rated temperature, etc. A bigger one is not necessarily better than a smaller one.
Most of the work is done by the semiconductors - transistors and diodes. The diodes in particular run huge currents - potentially 50A peak. If you have access to a parts catalogue and can look up the specs of the semiconductors then this would be ideal - as bigger diodes mean higher efficiency, lower heat production and better voltage stability. Failing that - look at the size of the heatsinks. Big heatsinks mean that the PSU is designed to operate under adverse conditions (high ambient temperature, high power).
Don't forget the fan(s). Look for quality ball-bearing fans designed for operation at high temperature. I've lost a number of PSUs because of failed fans. Multi-fan PSUs provide at least a bit of redundancy if one fails.
Other things to look at which are also very important are the low-voltage wires and the quality of the connectors. I've seen a number of motherboards/PSUs with melted molex connectors on (usually the 20 pin one) - often because of low-quality clone parts on both motherboard and PSU. Some manufacturers now use name brand parts, some even with gold plated contacts. The other thing is size of the wires - there can be a significant voltage drop along the connectors from the PSU which is compounded by the fact that the voltages are so low to start with - thicker is better - although most manufacturers do use the same size - some budget brands may use cheaper wire.
I've chaged my view on what to look for in a PSU several times as motherboards and periphs have changed. My current view is that you should look for a PSU with the highest rated 12V rail that you want. +5V and 3.3V are secondary to this. I'm less fussed about regulation quality now than I was a couple of years ago - the reason, virtually every peripheral or major subsystem on the motherboard now has its own voltage regulator (CPU, RAM, AGP slot, graphics card, drives) which is tailored to the device's requirements, and can do a far better job than a generic PSU - not least ebcause there isn't a foot of wire in between the regulator and load.
Some manufacturers have now started producing supplies with individually regulated lines (e.g. Antec True power) with seperate regulation on 12V and 5V. On conventional PSUs the average of the 12V and +5V lines is regulated. A nice idea, but higher cost, and these days probably doesn't offer any real benefit.
Some manufacturers have produced supplies with multiple 12V lines (e.g. Enermax) - one for the mobo and one for periphs. An interesting idea, because the mobo 12V line is subject to huge current swings as the CPU operates - this in turn causes the voltage to fluctuate on the rail. This permits the peripherals to get cleaner power. Again, an interesting idea, but probably doesn't offer any real benefit (I'm not sure about cost - it could go either way).
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