It's a PCIe 1.0 4x port if I'm not mistaken. If I remember it right, PCIe 3.0 cards needs at least a 1.1 port for compatibility.
I wouldn't be so sure, that this is the case. Breakage of backward compatibility already happens on PCIe 2.1 (at least according the wiki), but PCIe 2.1 cards are working on Dual boards with no issues.
"PCI Express 2.1 supports a large proportion of the management, support, and troubleshooting systems planned for full implementation in PCI Express 3.0. However, the speed is the same as PCI Express 2.0. The increase in power from the slot breaks backward compatibility between PCI Express 2.1 cards and some older motherboards with 1.0/1.0a, but most motherboards with PCI Express 1.1 connectors are provided with a BIOS update by their manufacturers through utilities to support backward compatibility of cards with PCIe 2.1."
Anyway, I would say that even those cards are plenty powerful, especially if you're considering them for XP gaming. They're also supporting DX11 (even DX12, you're considering that Fermi implementation as a real support) and in combination with some C2Q you can play many early DX11 games on that machine in Win 7 with no issues.
Socket 775 - ASRock 4CoreDual-VSTA, Pentium E6500K, 4GB RAM, Radeon X800 XT, ESS Solo-1, Win 98/XP
Socket A - ASRock K7S41GX, AMD Athlon XP 3000+, 512MB RAM, GeForce4 Ti4200, SB Live!, Win 98