Yes they have changed the models, go for the NF-R8 Redux 1800rpm, they are all quiet and the air flow over the 1200rpm one will be needed if you have an internal drive fitted.
I think the fans I ordered are probably not compatible. They are a PWM version and use 4 wires. I have returned them & plan to get the 3 wire version. I notice that the original fan is a 2 wire version. Does the connector on the 3 wire version just plug onto the MX mother board or is rewiring necessary ?
I can't remember it was a while back, I think I may have cut the plug from the original fan and soldered the wires onto the cable from the new one, there may be an adaptor in the box.
If I remember right on the 3 pin fans the red and black are the power cables just like the 2 pin fans, and the yellow is for speed control, so you don't use the yellow and the fan runs at full speed, which is what you want.