@ Miles - totally right - it's the current on the +12V (and +5V to a smaller degree) line that's important.
Massive overall wattage is a misnomer. If you've got a bog standard, or even a reasonably expanded system - then forget an expensive ATX PSU - get a good quality secondhand(i.e. from an old Dell, HP, IBM, WHY) AT or ATX at a fraction of the price.
I use an AT PSU that's over ten years old and 'only' 200W.
Let's compare that to a brick:
Brick
Rating @ 60W
+12V @ 1.0A
+5V @ 4.3A
-12V @ 0.1A
AT
Rating @ 200W
+12V @ 8.0A
+5V @ 20A
-12V @ 0.5A
For big tower systems then a bigger PSU may well be needed, but again a good quality 400W (i.e. Enermax) PSU will prove better than a cheap 500+ watt jobby.
