The Asus P5GC-MX/1333 motherboard, uses an Intel 945GC motherboard chipset,http://www.asus.com/Motherboards/P5GCMX1333/http://en.wikipedia.org/wiki/List_of_intel_chipsets#Core_2_chipsets
When installing in the PCI Express x16 slot on the motherboard, (Black. Gently lift up on Locking Arm to install card. Looks like you press down), there looks to be enough room.
There are Radial Aluminum Electrolytic Capacitors that stick up, on one side; and one of the white PCI slots on the other side.
Nvidia GeForce GTX 560 is a wide graphics card,http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=7758758&CatId=3669
Just one manufacturer. Size varies a little from manufacturer to manufacturer.
Minimum power requirement is 450 Watts.
Now bear in mind that this is for a complete computer system.
NOT, what just the graphics card requires for power.
Since there is no additional power cable to be connected, to the graphics card, the graphics card ONLY uses UP TO 75 Watts.
This is known because a PCI-Express x16 slot, will deliver UP TO 75 Watts.
If a graphics card requires more power, then a 6-pin PCI Express power cable is used. It will deliver an additional 75 Watts, bring the available power for the graphics card UP TO 150 Watts.http://www.playtool.com/pages/psuconnectors/connectors.html#pciexpress
Graphics card requires more power than that?
Then an 8-pin PCI Express power cable is used. This power cable can deliver UP TO 150 Watts.
With PCI Express x16 slot, you are now looking at 225 available Watts,http://www.playtool.com/pages/psuconnectors/connectors.html#pciexpress8
[NOT to be confused with the 8-pin EPS +12 Volt power cable. This power cable is used for the MOTHERBOARD, NOT a graphics card,http://www.playtool.com/pages/psuconnectors/connectors.html#eps8
The connector on the Top of the graphics card, is for SLI.
Scalable Link Interface,http://en.wikipedia.org/wiki/Scalable_Link_Interface