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
edited to add quote.