at the moment, i have one of the first surround processors put on the market (yamaha SR-50), and i will change it in the future for something better (thinking about the Integra DHC-40.2).
i will also need a 2 channel amp for the back speakers; the whole system will be 4.1

i think the Behringer EPQ450 would be ok for power, but i would like to have some opinions about the quality and durability of that company's amps when used in a a/v system.

Not much gets sent to surround speakers except maybe during music videos, even then it isn't the same low frequency stuff that demands high power amps. Behringer has some decent amps from some reviews with some comments about bad/noisy gain controls.

For back surrounds, I'd consider grabbing a Sonic Impact T-Amp and external power supply for about $60 and save yourself a bunch of money.

This amp at Parts Express is the same one for $34 and I think it includes the power supply.
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-380
but I would recommend going one step up to this one which has more power for $99.
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-383

I've almost decided that it will be Emotiva UPA-2 for the 2 back channels;
any comments will be much appreciated from people who own a UPA amp?

this amp should give me some spare watts for peaks so it never clips any waveforms, just like my Hafler does, right?
it will be used in conjunction with Energy 22 Ref. spkrs. (anechoic: 86 Db).