Yes biamping with a powered sub will be a great improvement. I know from experience. However be prepared to also purchase an active crossover like a Marchand or Bryston or maybe even one of the digital room correction pieces with a built in crossover to analyze your room. Then, unless you can afford to pay someone who really knows the ins and outs of subwoofer/crossover setup be prepared for lots of tweaking which will include subwoofer placement, finding the modes and dips in your room's frequency response, dialing in the proper crossover points and slope, and of course proper placement of your listening position and main speaker position. You can do all this yourself but it may take many hours of frustration. In the end though, the sound of your room/system will improve astonomically. Also be prepared for the added expense of interconnects that will match the level of your current systems needs.
Before you start this project make sure you've got proper bass trapping through out your room as nothing you do above will make as profound a difference as this.
I have gained this knowledge through my own experiences.
As to your other questions, yes vertically biamping will help and you'll here a significant difference. However it will not transform your main speakers into separate subs and mid/high frequency speakers. I've done this, (and still do) but the addition of subs powered with their own amps is the better way to go, if your goal is to transform your system from great, to something that you can really be proud to own.
I'd be glad to share further insights in depth if you'd like. Talking is easier than typing.
Hope this helps,