diff b/w modular & nonmod psu and multi 12v question

1) With modular PSU's most of the cables can be disconnected which allows you to only use the cables you need. The problem with this type of connection is it's increased resistance and it's not as secure a connection as the cables being hardwired. This is a hotly debated topic and there are many on both sides of the issue, I prefer the security of my cables being permanantly connected and I don't have a problem tucking away the cables I don't use.

2) There is a disagreement between manufacturers as to the need for multiple rails but I've found that the biggest reason they use more than one is that it allows them to use cheaper components due to the fact that there is less current flowing through them. I beleive it is better to have one high current, high quality rail rather than several lesser performing rails but others may disagree.

3) When using multiple rails the total amps are less than the sum of the rails. Take for instance my PSU the Thermatake Toughpower 750 which has 4 12v rails rated at 18a each but my total for the 12v rails is 60a. All PSU's should list the total current for each rail as well as all rails combined and if the one your looking at doesn't I would look somewhere else.

I realize I suggested a single rail PSU and I myself am not using one but mine is still a quality unit and I got a great deal on it. All things being equal I would choose the single over the multiple rail unit.

You can use this to find out about how many watts you need.
http://www.extreme.outervision.com [...] orlite.jsp

As AUsch30 said a single 12v rail is better than multiple 12v rails, but good brand psu's with multiple 12v rails are still good.

No, That's recommended PSU size. Here is some information for you to read and a good price on a top quality PSU.
Power Supply Myths Exposed!
Silencer® 610 EPS12V Power Supply