You could always try some interpolation software to increase both the pixel density and the physical size. I've used Alien Skin's 'Blow Up 2' with some reasonable results for my 'arty' stuff. Not tried it out on anything as exacting as a photo though.
Try out the demo from here>>>>>>>
http://www.alienskin.com/blowup/
__________________
"I meant," said Ipslore bitterly, "what is there in this world that truly makes living worthwhile?"
Death thought about it.
"Cats," he said eventually. "Cats are nice."