What would the gaming performance of a virtual PC running Windows XP be like?

That's generally a very open ended question...depending on your virtualisation method. I use VMWare workstation for that type of thing and get approx 2/3rds the performance compared with running games natively.

A free option that supports 3D rendering would be VirtualBox, but I haven't used it in a long time to compare performance.

When playing games, I've found it very hit and miss what works well...some games will run amazingly well, others will be choppy as hell. Can't hurt to try though.

Solitaire works quite well.

...U'd get better graphical performance from a sega master system