What power supply to get

[orangat]

The psu is one of the most important components of a pc. A faulty marginal psu can be the source of hard to diagnose borderline problems including crashing and data corruption. It can also damage other expensive components like the cpu/cards/mb. There should be more emphasis on getting a quality unit instead of being contented with the freebie that come with the case.

Obsolete pcs-
Old systems may require the -5V rail. It will not need a strong 12V but a strong 5V rail instead. Any psu post ATX12V v1.2 may not supply -5V although many still do I think.

Super high end sli-
All dual rail psus no matter how high the wattage which follow the ATX12V specification to the letter will not be able to support sli/xfire with high end cards like the x1900xt. A psu with single rail or tri/quad rail would be needed unless the manufacturer ignores the spec and allows more than 18-20A on a single rail.

Typical systems-
The capacity of the 12V rails is the most important factor when sizing psus because newer cpus and video cards practically draw everything from the 12V rail. The typical draw from the 3.3V/5V rails should be around a measly 30W combined. A high end P-D cpu like the 820 can pull as much as ~11A on the 12V rail (135W). Dual rails on smaller psu's - anything less than ~350W would be pointless since it limits flexibility. A single rail would do.

Quality-
I don't know where Antec got its reputation for good quality psus from since it still uses very low quality fuhjyyu caps in all of its lines including the NeoHE which is oem'd by Seasonic. Despite the glowing 5-star reviews in newegg, this is a brand to stay away from. Delta, Etasis, Zippy/emacs are top quality brands. Pcpower and cooling, seasonic, enermax are very good. Enhance, Fortron/Sparkle (FSP models, not AX/ATX) are quite alright. This is just a short list of decent psus and there is some overlap since some manufacturers do put out lower quality price leaders.

The bad-
These psus are pure junk with poor build quality and components. Listed are some of the worst manufacturers and their oem labels.
Deer - Codegen, L&C, Mercury
TTGI - Superflower
Youngyear - Xconnect, Aspire
This thread lists more psus/manufacturers. Its not a static list, some of the manufacturers can make quality psus. link

Standards
ATX v1.3 and below are for obsolete pcs with the 20pin main plug.
ATX12V v2.x specify a 24pin power socket with an additional 4pin cpu power socket.
EPS12V/SSI is a server spec but the main difference from ATX12V is the 8pin cpu power socket.
Using an old ATX v1.x psu for anything newer than P3/Athlon is asking for trouble unless the 12V rail is beefy enough.