A power supply question

I have an Acer Aspire desktop that has an AMD 4400 dual core with 3gigs of ram, Model M5100. The power supply is a 300 watt that has a 20 pin plug, but it is plugged into the motherboard that has a 24 pin socket, so that the last two rows of two sockets are empty. This MB has built-in video, but it also has a PCIe slot. I have a spare SEASONIC 430watt PS that has a solid-not split, 24 pin plug. It also has a PCIe plug. I would like to use this Seasonic PS in the ACER because I also have a GeForce 7900GS PCIe video card that I had outfitted with a Zalman cooling fan, and would like to put it into the PCIe slot on the ACER MB. The video card has the six pin PCIe socket. I wonder what the consequence of my plugging the 24 pin plug from the Seasonic into the ACER MB is. Should I first cut or snip the last 4 wires? I hate to approach ACER on this because I'm sure they'll tell me all about the voided warranty that would result.
I also have a PCIe video card that is an XFX GeForce 8400GS that is cooled by only a heatsink. Does the PCIe plug from the power supply merely supply power to the cooling fan on the video card, or does it do more than that? I've often wondered why some high end gaming video cards state that the computer should have a minimum of 450, sometimes 550 watts in the PS. Surely there is more at play here than just spinning a 2 or 3 inch fan. Thanks.

 

You shouldn't have a problem plugging that 24 pin main header into the 24 pin power recepticle on the motherboard. And, as to your second question re: the PCIe molex connector - it depends on the video card. Some power only a fan, others power not only a fan, but more.

 

In regards to your power question PCIe cards are usually heavy power users, on a rough guess your 300W PS running a AMD 4400 dual plus that PCIe card would be very much underpowered.
I would say that the 430W SEASONIC would be just about close to minimum.
There are some web sites where you can check your requirements you enter your hardware configurations CPU - Video -HDD - DVD Rom and so on and it will calculate your power needs.
hawk22

 

Thanks to one and all. I abandoned the project as it turned out the 7900GS with a Zalman cooling fan is just too bulky for the ACER case. I do have a low profile 8400GS with just a heatsink on it, and it fits the PCIe slot nicely. So, I put that in, and it seems to be an improvement over the ATI chip that is on the motherboard. So, I'll live with that.
Thanks again.