There are certainly a few hints that Tyrion could be a Targ through King Aerys and Joanna Lannister having an affair, or the king forcing himself on her. Not good evidence like with R+L=J, but you can make a case. Tyrion is obsessed with dragons, has no fear of fire, has white blond hair like a Targ instead of golden, has one eye that is black (Targ eyes often look black until they show as dark purple in the sun), etc., plus Selmy tells Dany that there was something there between Aerys and Joanna.
Of course you could also make a case for Jaime and Cersei to be Targs.
A thread on the topic of Tyrion as a Targ at westeros.org is linked here:
http://asoiaf.westeros.org/index.php...rgaryen/page-3
And either Tyrion or Jaime will kill Cersei, that much seems to be a given based on the prophecy Cersei received that has been pretty accurate so far. Cersei certainly believes it will be Tyrion as the "Valonquar" or little brother who kills her, but Jaime was born after Cersei, so he could technically fit the prohecy too, and for whatever reason he seems the more likely candidate to me.
You can read up on Cersei's prophecy from Maggy the Frog here:
http://awoiaf.westeros.org/index.php/Maggy