. . . He pushed the pioneer elite vsx-21txh receiver on me since it was in my price range of 500-800 bucks. Then he told me to take the rear left and right channels that I wasn't using and to wire those terminals to the other 2 unused plugs on the towers to get better sound out of my main L and R loudspeakers.
Haaspac,
For $500-800, there are many other excellent AVR options, recommend you independently research vice "being pushed". Check out units from Denon, Onkyo, Yamaha, or HK (just to name a few).
Is that bi-amping or bi-wiring and what kind of gain would I expect from doing that?
Bi-amping (sort of). "True" bi-amping involves using an active crossover and independent amplifiers, which do not share the same power supply (unlike what the Pio would do). I would not expect much, if any, noticeable improvement.
However, since it costs nearly nothing to try (a couple pieces of speaker wire and some time to properly set your AVR) give it a shot if you like.
If you do, be sure to follow the speaker and receiver manufacturer's WARNING about removing the "bridge piece" of metal on your speakers between the two sets of speaker wire terminals, or you will be very sad
Or would buying a separate 2 channel amp from emotiva or the like be the way to go?
Yes, but obviously costs a lot more, and unless you get an amp with close to double your receiver's actual power (discussion for a different day!

) you still may not notice much improvement.
Having your mains on a separate power supply (i.e. the Emotiva) reduces the burden on your AVR which, for most brands, improves the available power to the remaining 3.1 channels.
For more info on bi-amp and bi-wiring, here is a decent technical article
(courtesy of a fellow Audioholic "3db")
http://sound.westhost.com/bi-amp.htm
Cheers,
XEagleDriver