cant figure out which models have tahiti and which have pitcairn

i bought a new 7870 to crossfire and i couldnt get crossifre to activate. then i opened GPU-z and discovered they changed the gpu on them.... so now i want to buy another card to replace it but i cant figure out which cards have witch GPU.

i bought
SAPPHIRE 100354OC-2L Radeon HD 7870 GHz Edition 2GB and that has the newer pitcarn gpu in it. id like to just get another with that one with pitcarn. i cant find anything online to figure it out. if you guys could help that would be

i hope that this one on newegg will match up to it
http://www.ebay.com/itm/380897181889?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

Pitcairn is the original core for 7800 series amd cards. They later released a 7870xt that was a Tahiti core with more units shut down than a 7950. It was stupid of them not to call it a 7930.

If the card has 1536 shaders it is Tahiti based and will xfire with 7900 series cards. If it has 1280 shaders it is pitcairn based and will xfire with 7800 series cards.

That sapphire on eBay is pitcairn based.

Edit: for what it's worth, the Tahiti 7870s are faster cards. If you already have one of each, and the price is similar, you would be better off finding a second Tahiti based 7870