The STF2 is a good sub, and it is definitely not overkill for those speakers. My question is how loud do you like to listen? That is a good system for a modest volume, but if you want to watch movies at sound levels commensurate with that of commercial cinemas, my advice is to beef up the subwoofer and skip the center to begin with. You can also get a higher quality system and save money by using bookshelf speakers for the front stage, as you do not need the deeper bass extension of tower speakers when a subwoofer is tackling those regions. I am saying a better foundation for improvement would be to blow your budget on a 2.1 with the speakers as bookshelf speakers. If you want the power of tower speakers later, you can always use the bookshelfs as surrounds when you upgrade.
Get some bookshelfs like these
HTD Levels Threes,
Ascend CBM-170s,
Hsu HB-1 mk2, or these
Arx A1b, and put them together with this
Hsu VTF2 mk4 sub, that sets you up for a great system.
If you want a
really solid foundation for a great system down the road, just go with 2.0 for now, like
these Focals for instance, or these
KEF towers. Also worth considering is these
Arx A5 towers and
these EMPtek E55Ti towers. Towers can get you you some decent bass until you get a subwoofer. If you can do without some solid bass for awhile, getting a 2.0 of high quality bookshelf speakers would be setting yourself up for an even higher end system eventually- check out these
Ascend Sierras, or these
KEF R100s if you want something really hi-fi. Of course, higher end components make the upgrade path a longer one in that you want to keep everything to about the same level of quality, but the more you invest individually in high quality parts, the better system you will have and the longer you will have it for the pride of ownership. If you buy cheaper stuff now, it will be a lot more disposable as you will be less likely to care for it to the degree that you would something really nice.
Anyway, spending $1k on a couple of bookshelf speakers may be absurd depending on your goals, but I am just mentioning that as a possibility since you are open to the idea of building up your system. If you don't need really exquisite sound or bass that will pound your chest, your listed setup would be fine. As for receivers, there are a lot of good options out there, I would be looking at Yamaha, Denon, Pioneer, Harman/Kardon and maybe Onkyo. A good place for refurbished receivers is
Accessories4less.com.