As a suggestion, you could go to a website, like NewEgg, to look at server motherboards. You could then see the embedded GPU chipsets those mobos have. All of these will be supported, in all likelihood by the server OSes you are inquiring about.
Also, both OSes will use just about any GPU with standard VGA drivers. The GPU prowess of most servers really isn't an issue since they are usually only used when maintaining the system. I typically configure my servers via remote desktop.