T1M0N6 :
So you're tell me that if I were to buy a Radeon R9 series I couldnt have them with my HD7750?
Exactly. Crossfire means that every other screen line is computed by another card. First card computes the first line, second card the second, first card the third, and so on. As the consequence, each of the card only needs to do half the work and can be correspondingly faster. More consequences are that your GPU power consumption and heat dissipation are doubled, that you need another long PCI-E slot (usually along with adjacent slots since most graphics cards are at least 2 slots wide), and that there are sometimes flickering issues that come from synchronizing the results from both cards.
I think it is pretty obvious that this system only works with two identical GPUs.
So it is a possible way to prolong the life of an old card for low money, but IMHO not that awesome.
As an exception to the rule, some Radeon cards can crossfire with the on-CPU-graphics of certain AMD CPUs, but the results are reported to be disappointing. You hardly gain anything.