A tablet PC is useless for retro gaming due to lack of any gaming controls so if you want a retro gaming handheld a tablet is worthless. If you consider balancing a tablet on your lap while using a wireless controller a handheld then take alexh's advice. If you want something that can emulate up to N64 and PSX then you want a pandora. Or you could buy an IcontrolPad that clamps to your apple or android phone and get your retro gaming on that way.
http://www.openpandora.org/
http://www.icontrolpad.com/