I'm going to base my answer on the Aspire M5800 series.
If you're able to provide a part number for the system, I can verify the model.
Based on your post, you're looking at installing a SATA 3 PCI card and not a SATA 3 PCI Express card, is that correct?
Specifications for the Aspire M5800 system indicate the following slots:
1x PCI Express Slot Type: x16
1x PCI Express Slot Type: x1
2x PCI Slot Type: x1
Just in case, the PCI Express slots are 1.1 and not 2.0.
Hope this helps.
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I have the same Acer EG43M mobo in my M5800, I'm wondering what I need to know for compatability issues when I'm upgrading the video card? I know the pci-e slot is 16 pin, but SATA 3 is that the power input for the card to plug into? I want to put a gtx 560 or 660 in to this pc, since the 2.0 cards are backwards compatable it will work I'm hoping..