Personally I think you should consider using a pair of rena xp-3 filters or if you want to stick with a single filter you can use a rena xp-4.
The FX5 is a computerized "microprocessor" filter, like the Eheim Pro's that has an adjustable flow rate & other "bell's & whistle's"most of the time LFS recommend them to 200-400gl if they're brave enough to even carry them
but you can def use them for a 120 for sure. I run 1 FX5 on a 75gl (in addition to a total of 8 other type's of filtration on this tank)cause & here's my answer to the "overkill ?" if your fish can hang then you can NEVER HAVE TO MUCH GOOD FILTRATION. I live to be able to provide 3-5x's or more gph of filtration than the tank size i.e. my 75gl has a total of 1600gph of filtration
And my African CICH's love it
Swim or Die is 1 of my mottoes. Between flow & aeration most of my tank's are like surf/braker zone's. Not like their natural habitat's, but they do great w/ it & allow for greater bioload's. I don't employ this method w/ my Angle's
Keep us up to date on what you go w/ & where you get it from. Sure you know, but do not buy w/ out chking online, example's (http://www.drsfostersmith.com/product/NavResults.cfm?N=2004+22777) & (http://www.aquariumsuppliesforless.com/) among other's.
Response to:
Would a Fluval FX5 be total overkill on a 120g? I'm kind of thinking about getting a tank this size (since I can't possibly fit a bigger one in my room)