I set up a 450 about a year ago and I went with the combination of canister filters (FX5's in your case) and a sump/overflow system. It seems to work great. I set up the canisters to push/pull water from the lower areas. This seems to keep the sand on the bottom of the tank clean by stirring up the lower parts of the tank and providing crystal clear mechanical filtration. The sump/overflow system provides some mechanical filtration with the added mega bio filtration and extra water capacity where I can hide my heaters.
IMO: I would keep the FX5's as a supplement to a primary sump/overflow filter. If your just going with one filter, go with the sump/overflow type. That's the most efficient type of filtering for larger tanks.
