well, that depends. I also have an Antec 900. Which way is the cooler facing? Is the fan blowing air from front-to-back (or vice versa) or down-to-up (or vice versa)? In my case, I have only expansion card plugged in, it's my video card. If I remove the video card from the case, I can remove one side of the Xigma's rubber fan grips, and from there I can reach all 4 screws on the retention bracket. I believe the only way you can install the Xigma that would force you to remove the entire motherboard is if you faced it to where it was blowing air straight to the bottom of the case, which would make the fan right below the 200mm Big Boy case fan.I must've applied a lot. I guess i'll try the 2 half line method (the very last 2 pictures). Oh man, that means that i'm gonna have to disconnect everything, including the mobo since the Xig is too big that it tucks underneath the Antec case.