right but amps handles also has to do with wire gauge, and I am fairly sure they are only speced to handle 60watts thru the 4pin motherboard connector.  I konw the 6pin connector is speced to handle upto 75watts on most power supplies.

You know... good specs on this stuff are surprisingly hard to find.  However, being that both ATX12V and EPS12V are speced for power supplies >400W so I'm fairly certain that the ATX4P and EPS can handle more than 60 and 75 watts respectively.  Now, you may be thinking of the PCI-E graphics card 6 and 8 pin plugs.  Those are rated for up to 75 watts on the 6 pin version, and 150W on the 8 pin version.  Those are potentially limited more on PCB side rather than current carrying side, but that's not for certain.  Just doing the math on 18 gauge wire (pretty standard power supply wire) and you're looking at over 10A of current at 12V per wire.  Technically the wire can carry 13A without burning up, but I wouldn't want to get near that number... So at 10A per wire that means a theoretical maximum of 240W for a pair of wires.  Would I run 240W through two wires?  Hell no!  But at 150W that's a good enough safety margin. 

Whew!  After all that (and reading some other posts) I may at some point just break out the bed heater and extruder heater section to its own daughter board and run that off a 8 pin EPS plug.  Doing that means I could potentially add two more extruder heaters later without worrying about hitting any max power limits.  Call that the belts and braces approach.  Till then though, I'll keep an eye (and a fire extinguisher) out for any issues with the ATX4P setup.

that ATX 4 plug is only rated for 60watts of power.  Be careful connecting your PSU with just that one cable.

From what I've researched the current carrying capabilities of molex connector vary between 5A and 8.5A depending on the width of the pin.  I'm pretty sure my pins are not the smallest size (1.5mm) so its going to be rated to carry around 6 or 7A per pin (assuming its not the max sized 2.36mm pin), or 12-14A total.  This gives a wattage of 144W - 168W.  Currently The system  (with stock power supply) draws up to 120W from the wall (so its gonna be less than that through the 12V line).  I'm expecting current draw to go up to 150W with just a bed upgrade and maybe 180W with both a bed upgrade and a heater cartridge upgrade.  At that point I'll see if those ATX wires are still holding up ok.  Anecdotally, (and based on some spec sheet math) I should in theory be able to push 185W through the ATX4P connector under ideal conditions.  I think at that point the limiting factor will not be be connector or the wires but the Sanguinolulu PCB itself...

One of the first series of mods I'm doing is with the power system.  People have gone with various methods of increasing power in order to run hotter beds and extruders, but I'm trying to get something that uses the fewest bare wires possible while gaining the most amount of control (I'll explain later).  The first mod I did was swap out the screw terminals for power with a female ATX4 plug.  This will allow me to run a standard ATX psu.  The next mod will be removing the 7805 regular and instead running the 5V side from the 5VSB of the ATX power supply.  With those combined I can then tie a spare pin to ATX_ON and have an always on SD3 that will automatically power on the main system when a print starts and shut itself down when a print stops.  From there on out the sky's the limit.  Here's a small list of upcoming items, not including new printer parts, which are a totally different topic:

- actively cooled Sanguinolulu with a 80mm fan and heatsinks on stepper drivers (finishing scad model for mount)
- QU-BD bed heater (need to get some acrylic laser cut for a custom base)
- 40W Ceramic cartridge heater for extruder (still shipping from China)
- Moving from a full size ATX PSU to a rackmount 1U PSU in order to mount the PSU to the back of the SD3 (need to find PSU with sufficient power/size/price)
- Active cooling all motors

Update 1:
I found my power supply!  I was looking at a bunch of 1U supplies when a coworker pointed out that hey, 1U supplies on load will probably sound like a small jet engine.  He (also a recent 3d printer purchaser) recommended a FTX supply, which is what goes in a lot of ITX cases.  Looking around I couldn't find a good used FTX supply, but what I did find might be even better.  I bring you the Seasonic SS-460H2U.  A quick rundown of specs: 2U sized, 460W, fully modular, active PFC, and 80 Plus rated.  All for $15 from Weird Stuff.   I love that place