It's simple math. The 18 gauge stranded wire used for the power cables are capable of passing about 10 amps of current through them. So now, you just add up how much current each device draws from the 5 and 12 volt lines.
A typical fan draws 0.25 amps from the 12 volt line, and none from the 5 volt line. So we're up to 0.75 amps in your case.
At the auxiliary power connector, the NVIDIA GeForce 6800 draws approximately 1.61 amps from the 5 volt line at load, and 2.01 amps from the 12 volt line at load.
Adding this all up, we get
1.61 amps flowing through the 5 volt line, and
2.76 amps flowing through the 12 volt line
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