A passivly cooled 7800gt is gonna heat everything else up anyway. Your going to need to have some airflow going it its direction, or that thing is gonna get toasty warm. If your going AMD, you've also gotta remember that the NF4 chipset will most likely be sitting under the GFX card. You dont want that getting too warm.
My pc's reallyquiet, and iv got an oc'd 7800gtx on it with stock cooler. Unless you stick your ear up to the side of the case, you cant here the cooler on the GTX. The thing still gets damm hot, so i dont even want to think about how hot a passivly cooled 7800 would get.