The Intel G31 Express Chipset is PCIe v 1.1 period, so there's no BIOS to make it anything more. The HD 7750 is going to be bottlenecked by your LGA 775 CPU to some degree, and the only question is by 'how much.' PCIe 3.0 is backwards compatible, at least on paper, but I haven't seen anyone on a PCIe 1.X MOBO's running PCIe 3.0 GPUs.
Therefore, you're a Guinea Pig. My best guess is the HD 7750 should work but don't expect that 'advertised' or 'reviewed' frame rates.