Chaingunchris :
if I add a 4gb 270x in replacement of my 2gb that's in there already will the games detect the 4gb card or the 2gb card
Wait, in replacement of? Or in addition to?
If you're adding a 4 GB card to pair with a 2 GB card in Crossfire, the lower of the two sets the limit for both cards: 2 GB.
If you're replacing a single 2 GB card with a single 4 GB card, then yes of course the 4 GB is usable (though it rarely will be used).