If it has a PCI-E x16 slot it supports all modern GPUs
Not true, The card he gets must be compatible with the version of PCIe slot he has.
ie - A GPU running PCIe v2.1 might not work in a PCIe v1.0 slot. The Galaxy gtx580 I just looked at is one such card. Its only backwards compatible to PCIe v2.0
my dad has a pcie 2.1 ati 5570 vid card with that motherboard working perfectly. So it will be compatable with pcie2.1 . There is also a BIOS update for the motherboard that fixes a problem with new ati cards taking a long time to initialise. I also have the same motherboard butt with an 8800gts. I would still not bother putting such a high end card in that mobo, it will be bottlenecked by whatever cpu you have unless you have an overclocked 4ghz core 2 quad. Im thinking of making a thread for a compatability list of pcie 1.x motherboards and 2.1 vid cards. In reality the pcie system is designed to be backwards compatable, but manufacturers that dont release a BIOS to support it is the problem.