The reason that I recommended the B2030P and an old used receiver was that the amplifiers that are built into the B2030A have a so-so reputation for reliability. Keep in mind that's third hand information so take it for what it's worth. The second reason is bass management.
http://diplo-best.com/review/BEHRINGER/B2030A
Sound cards: If you go with a receiver and passive speakers then the sound card isn't all that important as long as it has a digital output. The reason is that it's just passing bits to the receiver and the receiver is doing any decoding the high quality DACs in the receiver are doing the digital to analog conversion. If you go with a good powered speakers then you'll want a sound card with good quality DACs and a good clean preamp section. Both Asus and HT:Omega make top quality internal cards for PCs for around $170ish. I have no idea about external or Mac cards.