Well I have been looking at this for last few days in detail.
first what the heck are you running. specificlay the video card as thats usually the biggest hog.
Personlay I am looking at the 8800GTX video card which MIn is 450 watt so I am shooting for the 700 Watt range.
now there are several things to look at here and some features are dependent on the price
1 is efficiency. there is a drive to have power supplies over 80% efficent. costs a little more but hey always good as it saves power bill.
so you want a active PFC, I do
here is a link to explain it
this is not required but a nice to have.
http://www.endpcnoise.com/cgi-bin/e/faqpfc.html
Next you deal with 12 V rails. There are oens with only 2 12 volt rails 3 and 4. 4 seams to becomming more common. Now each rail has a amp rating usually listed like the following
+3.3@36A,+5V@30A,+12V1@18A,+12V2@18A,+12V3@18A,+ 12V4@18A,-12V@0.8A,+5VSB@3.0A
This explains all outputs basically. Now see each 12 volt rail states 18 amps here, some systems have rails with as high as 30 amps per rail. and some have different amps per rail. Relay depends on the system. Now that being said the big thing as far as I can tell is stay in the 18 amp per rail or above area. There are a bunch at 15 amp and I just don't see this as enough for some power hungry cards. Now also note take a 8800GTX video card it says "Minimum recommended power supply with +12 Volt current rating of 28 Amps." Now that being said they mean total, so 4* 18 = 72. so again check the numbers here.
SLI is even more power reqirments but SLI also means a lot more cash to buy the 2 cards so I don't think you are headed this way.
Now the supply I am looking at for my enxt system is OCZ GameXStream 100Watt system. It has a lot of good reviews about it and seams like a good one and is a good price. It has all the stough I talk about above, it is SLI complaint. Tho they do not say it can run 2 8800GTX cards in SLI mode, it can run 2 8800GTS in SLI mode. that being said people have posted that they are using it on GTX 8800 cards inSLI mode, but I woul go bigger for that but hey give you a idea about the thing.
http://www.newegg.com/Product/Product.a ... 6817341002
There is also other things liek the Fan and the cables themselves. I want cables that are neat and tidy, some supplies, usually cheaper ones are a mess of cables. Blow is a pic of cables that are clean. See how the wires are banded together vs lots of lose individual wires.
Here is a messy one
Fan is another one, bigger fan usualy means cooler supply. the ocz comes with 120 mm fan always nice, o and its got blue LED's means nothing but looks cool. But there are ones with 80 mm and up.
I am not saying the OCZ is the one for you but is a great one and good for examples.
hope this helped