I put the 2 ohm Infinity replacement speakers on using the stock head unit. Later I put an Alpine HU and it also works good. The 2 ohm speakers are 2X efficient as 4 ohm, so I don't turn up the volume as high to get it loud.
If you know you are replacing the HU, just get the normal 4 ohm type. If you have no intention of changing the HU, get the 2 ohm type.

 

Raven- yea the speakers fit the bracket and everything, i just need to get a longer screw and im good.
Mike- yea your right ifi was thinking about getting a HU then i should have gotten 4ohm, yet i didnt know about all these differencies at that time. oh well.