My experience with Virtual Machines (Server 2008 Hyper-V) only used a virtual display adapter and did not have the full graphic card capabilities; making gaming impossible.
This Microsoft site
seems to suggest the same, but may offer the information you're looking for.
Windows XP Mode runs many older Windows XP productivity programs
As far as building an XP Only system, the best way (IMO) to determine whether or not a component supports XP is simply checking the manufacturer's web site to see if a particular component has drivers for Windows XP. (whether they will continue to support it no one knows).