As already mention in one of the vast number of your other threads a PCI-e 2.1 card may or may not work in a PCI-e 1.0 slot. It will likely work since there have only been a few reported cases.
Will a PCI-e 3.0 card work in a PCI-e 1.0 slot? That's unconfirmed since no one has tried it yet and report their success or failure.
Conservative choice is the HD 5670 since most time there has been no issues.
Or you can take leap of faith and try the HD 7750 knowing that you may have to return it.
where can i ask for help on the pcie 3.0 case? please help me