It that power supply says 400w but it lists 10A for 12v, then it's really a 200-250w power supply AT BEST.
Also, keep in mind that 3.3v and 5v are usually coupled together. I mean it may do 3.3v @ 20A and it should do 5v @ 20A but both together won't exceed about 100-120 watts.
Considering it says 10A on 12v, I would bet that the power supply actually uses 2 x 3A diodes as a 12v rectifier - if you want to use that 12v extensively, you may want to switch the diodes on 3.3v with the diodes on 12v, so that the 12v would be capable of more current.
Keep in mind that the labels LIE.
Here's for example an album with pictures I made of a 400w psu some time ago :
http://imgur.com/a/YFWcANotice it says : 3.3v @ 28A, 5v @ 40 A, 12v @ 17A but you look on the chips on the second heatsink, you see there SBL1640CT ( 16 A , 40v rectifier) used for 5v, SBL1040CT (10A, 40v for 3.3v) , and two 3A diodes for 12v in the center. Now in practice, as they're reasonably cooled, they may do a bit more current than rated, but still far away from the 40A or 28A values written on the label, not to mention the 17A for the 12v.
So you really have to open it up and inspect the parts carefully and determine the power supply's capabilities before taking those ratings as true.
Most power supplies already have some resistors on the output, but they're only meant to keep the power supply from going nuts without any load. These older power supplies, as they were designed for systems that used a lot of current on 5v. Without anything on 5v, the output voltage on 12v may be wacky.
As you may not use 5v when turning it on, you need to cheat and use a resistor to consume some current on 5v all the time.
You use Ohm's law : Voltage = Current x Resistance.
So let's take 10 ohm and 5v ... 5v = ? x 10 ohm => Current = 5/10 = 0.5 A
The power dissipated in a resistor is Power = Current x Current x Resistance , in your case Power = 0.5 x 0.5 x 10 = 2.5 watts which is below the 5w rating of the resistor so you're good (even though the resistor is rated for 5w, you normally don't want to dissipate more than 3.5-4 watts).