The ACX 2.0 isn't really an "ACX" cool in my opinion. But anyways, the only difference is that the ACX 2.0 has a single fan and the ACX 3.0 has the dual fans and better heatsink. Get the ACX 3.0 if you can.
The 6GB is worth it if you want to play modern day games above 1080P. Even at 1080P it's worth it to get the 6GB...just in case.
the difference is the size of the card and the pcb it is built on. the 2.0 card is only a single fan and has less power needs, while the 3.0 card is dual fan and uses more power.
2.0 card uses a 6-pin connection while the 3.0 uses an 8-pin. 3.0 card is also 94mm longer. also not the extra power phase for the 3.0 card (hence the extra power needs)
performance is pretty much the same with the single fan 2.0 card being a bit louder and warmer due to it's smaller size.
The ACX 2.0 isn't really an "ACX" cool in my opinion. But anyways, the only difference is that the ACX 2.0 has a single fan and the ACX 3.0 has the dual fans and better heatsink. Get the ACX 3.0 if you can.
The 6GB is worth it if you want to play modern day games above 1080P. Even at 1080P it's worth it to get the 6GB...just in case.