Power Supply Power Distribution?

PC power supplies use a rectifier and Switched Mode Power Supply to convert the high voltage AC line to an isolated high voltage internal DC line. This high voltage DC line then feeds several step down DC to DC converters (buck converters). The output of each of these DC step down converters forms what's called a "rail".

A "rail" is any power source which can ideally provide power at a constant voltage up to a certain current limit. If the current limit is exceeded, then the rail voltage will begin to drop. The mechanics about why this occurs is due to the way DC-DC converters work.

For safety reasons, it's not a good idea to have a large amount of current flowing across any one single wire. Current causes heat, and heat causes things to melt and catch fire. The ATX12v specification puts a power delivery limit of 240 volt amps on any one particular wire (although it was removed as of ATX12v 2.3, it's still a safety concern if done improperly) and PSU manufacturers have to take this into account. Modern high wattage PSUs can deliver well over 1,000 DC watts at 12 volts. On a single rail, this would far exceed the safety threshold specified by the ATX12v specification mentioned previously.

To get around this, manufacturers either ignore the requirement and build the PSU with thicker internal cables that can safely handle the extra current internally, or they employ multiple DC-DC converters for the same 12 volt supply level. The former, known as single rail, allows all 12 volt leads to draw power from a single large DC-DC converter but is more expensive to construct. The latter, known as multiple-rail, isolates the 12 volt leads into sections which cannot balance power but is easier to construct.

There are of course separate rails for 5 volts, 3.3 volts, 5 volt standby, and -12 volts, but these can be ignored and for the purposes of this post I will talk exclusively about multiple 12 volt rails.

Whenever you see a PSU that has multiple 12 volt rails, they will almost always deliver 20 amperes or less each in order to comply with the ATX12v current limit (despite it no longer being enforced). PSUs with a single 12 volt rail can deliver over 90 amperes on that single rail.

In multi-rail systems, the breakdown of how the rails are organized is usually written on the PSU itself or inside of the manual. In a system with two rails, one may feed the ATX connectors and peripherals, while the other feeds the PCIe connectors. In a system with 3 or more rails, they will usually have multiple dedicated PCIe rails.

Now, on to your problem specifically.

OCZ power supplies, specifically the ModXStream series, are crap. They suck plain and simple. You should use PCIe connectors for graphics cards whenever able. Using Molex to PCIe adapters is a bad idea and should only ever be used on low power cards and as a last resort. Replace your power supply with a good one from SeaSonic or XFX (XFX power supplies are manufactured by SeaSonic).

PCIe cards with a 16x connector (not necessarily running at 16x mode, just has to be full width) can draw 75 watts from the PCIe socket under PCIe 1.x and 150 watts under PCIe 2.0+ (although 150 watts is rarely drawn for compatibility purposes).

Your GTX 560 (assuming it's a standard edition) draws on average 150 watts, or approximately 12.5 amperes, under nominal conditions. This will be broken down into 75 watts from the socket and 75 watts from a 6 pin PCIe connector. Two connectors may be used to further reduce the load on the motherboard, sourcing almost all current from the PCIe connectors which are better suited for it. Combined, this is significantly less than the 18 amperes on one of your 12 volt rails and should not be causing a problem as long as it's not sharing a rail with your motherboard and CPU.

The XFX Pro series PSUs are a great choice.

Like I said above, OCZ power supplies are crap. It's entirely possible that the rail feeding your PCIe connector is just wonky.