It would depend on the particular case, but in general, if there isn't a case fan fairly close by (let's say 6") and the fan(s) are spinning slowly enough to be SPCR-quiet, then as long as there is venting above the PSU & space above the case where there hot air can rise and disperse, I see no problem. These cases would be perfectly suitable for the fanless PSUs discussed in this article: Antec Fusion, Fusion Remote, NSK 2480, Fusion Remote Max, Silverstone DG04/05. I am sure there are others; look for ones that allow the PSU to sit with its PCB down, as per Seasonic recommendation, and have venting above the PSU.
Just FYI, I'm using the X-400 in a GD04 (I believe that's the one; it's the one without the drive stealthing, not that it should matter) with a very similar build to the one here: 2500k, Kingston SSD, Samsung F4 2TB. No graphics card yet, but it stays up indefinitely with 4xPrime95 + Furmark. A Ceton and second hard drive will be added next month, and a Kepler graphics card whenever they come out and prices come down a bit.
At the point when those are added I'll have to be more careful about the airflow, which right now isn't really engineered at all (I can feel a faint breeze coming out the back of the PSU, and the tissue test confirms it). However, when the case is filled more the space in front of the left-hand side fan is going to be pretty stuffed and I'll probably remove the fan and block off that grill, along with the extra blank PCI slot above the PSU to force more of the positive air pressure out through the PSU.
I can take some pics if anyone wants to see, before or after the extra toys go in.
Edit: Also, no perceivable coil whine on mine, at any load I've put it under. The whole system has an audible low whoosh in the same room, at night, with no background noise, but no clicking or whining. It all gets drowned out as soon as the TV, refrigerator, wind, or anything else starts. I may undervolt the fans eventually, once I see the temperatures when everything is in.