You really should take that Power Supply, and use it on grandma's computer.
The 8-pin EPS +12 Volt power cable was brought out, to provide more power To the motherboard AND processor. http://www.playtool.com/pages/psuconnectors/connectors.html#eps8
The 4-pin ATX +12 Volt power cable has TWO yellow wires.
Yellow wires are 12 Volt wires. (And two Black ground wires) http://www.playtool.com/pages/psuconnectors/connectors.html#atx12v4
The 8-pin EPS +12 Volt power cable has FOUR 12 Volt wires.
In the motherboard manual, does it say, "Yes Tom. Go ahead and use a 4-pin ATX +12 Volt power cable. We don't care. We just use an 8-pin EPS for fun."
[ This is an 8-pin PCI Express power cable. Completely different,http://www.playtool.com/pages/psuconnectors/connectors.html#pciexpress8
The motherboard probably uses 150 Watts by itself.
No Ram Memory, no CPU, no fans, etc.
The CPU could use up to 125 Watts of power. Just depends on what AMD, socket AM3, processor you are using.
Now to graphics card's power;
The most Wattage a PCI-Express x16 slot can deliver is 75 Watts.
The most power a 6-pin PCI Express power cable can deliver is 75 Watts.
8-pin EPS +12 Volt power cable? 150 Watts.
When buying a Power Supply you should calculate all components needing power,http://extreme.outervision.com/psucalculatorlite.jsp
,then buy a Power Supply that has AT LEAST 10 percent more power than needed. Easier on the Power Supply.
Also a computer will NOT use more power than it needs.
10,000 Watt power supply, (Exaggeration ), and the computer only needs about 100 Watts for surfing the internet?
Computer ONLY uses 100 Watts.
Due to the price, the above, and the availability, you should use a 500 Watt power supply at least.http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=899123&CatId=1079http://www.amazon.com/StarTech-6in-Pin-Power-Adapter/dp/B002O21XHQ
Or use it on yours if it has enough Wattage.
Back in the day, the motherboard didn't need to supply that much power to components on it.
More powerful Processors, Ram Memory, and graphics cards, brought the power needed, up.
A 4-pin ATX +12 Volt power cable was added for the motherboard. Then 6-pin PCI Express power cable for graphics cards. Then the 8-pin PCI Express power cable for graphics cards. Lastly the 8-pin EPS +12 Volt power cable.
(Better make sure the Power Supply you have is good, if you wish to use the above power adapter cable. Nothing like having a Power Supply with a weak voltage power rail, and a new build, to pull your hair out on )
Out of the motherboard manual Page 23,
"With the use of the power connector, the power supply can supply enough stable power to all the components on the motherboard. Before connecting the power connector, first make sure the power supply is turned off and all devices are properly installed. The power connector possesses a foolproof design.
Connect the power supply cable to the power connector in the correct orientation. The 12V power connector mainly supplies power to the CPU. If the 12V power connector is not connected, the computer will not start.
To meet expansion requirements, it is recommended that a power supply that can withstand high power consumption be used (500W or greater). If a power supply is used that does not provide the required power, the result can lead to an unstable or unbootable system."
For additional questions please post in a Comment.