With the Antec 900, you have to double check the cables coming from the front of the case. The Antec 900 has cables that should be labeled correctly (if I recall, mine were labeled "USB"), and those cables need to be plugged into the board. There is documentation for this in your motherboard manual, more than likely.
As setishock mentioned, with this case, you want to face the PSU fan up. The fan will act as an intake, and will pull hot air from the GPU. There should be an additional fan in the back of the PSU that is acting as an exhaust, and so the PSU will not overheat. A PSU is going to have a high tolerance for heat, and will generally shut off before it burns out. Your GPU will usually not do this, so it's actually beneficial to have it work this way.
The Antec 900 has a lot of fans and filtered vents, and so cooling is probably not going to be a huge concern. I installed additional fans in my Antec 900 back in the day, by choice, because I wanted it to be pretty frosty inside. I also kept my fans at max speed (via the fan control), as I'd prefer loud fans over burning hardware.
Last tip for now (unless of course you have questions)... dust your case, often! At least once a month, or more depending on your home and pets, open the case and blow out the vents and fans. This case has so much airflow that any dust, hair, or impurities are going to get pulled in. I've had clientele in the past go a year or so without cleaning, and it was like a dog lived inside. The filters are going to catch a lot, but again, I like my cases to be super frosty.
Great choice on the case, and best of luck with the cable management! We all form different tips when it comes to cable management over time. My advice is to not only take what we suggest, but also take a look at how Youtubers have done it. For most of us, myself included, seeing it done in a video makes all the difference (and shows you the final product)!
