Well I'll try and give you a crash course Mmmmkay
Your priorities should be speakers first then recievers, because it is the speakers that are the most important factor in sound. The only thing we request is that you never push Bose speakers on anyone (mainly because they are crap and not worth what they charge and they cannot reproduce nearly 1/3 the audio spectrum that humans can hear. Otherwise have your customer test out a variety of speakers and choose what they like best.
Speakers range from 4ohms-16ohms (depending on brand) but the most common is 8ohm. The smaller the ohm the more power is needed to drive the speaker. You should always check what the reciever can properly handle (most are 6-8ohms) so you wouldnt want an 8ohm reciever driving 4ohm speakers, this may cause the reciever to overload.
In the 80's you could easily get three way tower speakers with a tweeter, mid range and a passive bass driver that was between 8-12" to handle the lower bass. Now you can easily get bookshelf speakers and a subwoofer to handle the entire audio spectrum (20hz -Hertz for low bass - 20kHz -Kilohertz for upper frequencies) Speaker companies are very misleading on what their speakers can do in the lower frequencies DefTech claims that there bookshelves can easily get down to 30Hz when they peter out at 60Hz for example. When selling bookshelf speakers, I would make sure that the mid range driver is at least 5.25" or larger so that it can easily can get down to 80Hz and blend with a sub.
Subs are normally crossed over at 80Hz (when you cross higher than bass becomes localized and you never want to know where the bass is coming from, bass should appear to come from all over the room. There are good subs and poor subs that stores sell, but the best subs are sold on the internet - No store bought sub can even match an internet sub as far as depth of bass and cost. Most store bought subs can get down to about 30hz before they start to peter out and alot of them sound boomy and have distortion. Basically when your in a demo room and a bass heavy scene comes on and you hear that loud grumbling of the subwoofer, that is distortion playing, deep bass should be more felt than heard and a great sub will sound very tight, clean and controlled during bass heavy scenes. For example Polk makes horrible subs while Klipsch and Velodyne are better.
Speakers and subs - The average human can hear sounds between 20Hz-20Khz as mentioned so you want the sub to be able to handle 20Hz - 80Hz (the crossover to the speakers) and you want the speaker to be able to handle 80hz-20Khz
In smaller rooms, I would easily recommend 5.1 setups and larger rooms 7.1 (.1 is the sub) The best scenario is matching speakers all the way around (including the center) this will give you the best timbre matching (sound is equal all the way around)
Recievers, okay most recievers with a certain price point will pretty much sound the same, it's at this point you will want to look at the different features that each brand offers and how they will appeal to the shopper. It's at this point I highly recommend you write down each brand and model number and read online professional reviews to get an unbiased review of what the recievers amp can do. With receivers the more speakers you hook up the less power is available to each speaker. So even though the reciever says it can handle 100 watts per channel, when you hook up more than 2 speakers that number drops dramatically. Sony for example says their reciever (The 2000.00 ES line) can handle 120 wpc but when 5 speakers were hooked up the Sony could only muster 45 wpc, with 7 speakers that number dropped to 41 wpc). Yamahas are just a tad better but not by much as far as power is concerned. Denon, Onlyo, Marantz and Pioneer fair much better in the watts per channel department.
Now Watts can be decieving, because 90% of the time your speaker is just chugging along at 2-5 watts at moderate levels, but during loud scenes in movies or rock music, for brief periods that speaker can require 60-80 watts in an avarage size room. Larger rooms require more power. Now bass requires the most power hence why the subwoofer will have it's own built in amp to help drive the woofer relieving the reciever from driving all that power and hence the reciever will have more power to drive the mid range and tweeter of each bookshelf.
Now DVD's and Blurays have all these codecs (sound audio encryption) and the higher up you go in bitrate the better the sound (depending on source mind you)
Dolby Digital is encoded at 448kbps (all ratings are generalized)
DTS is nearly double at 776kbps
Dolby Digital Plus is twice that of DTS at 1.5Mbps
DTS-MA, PCM Lossless, Dolby TruHD can vary but the normal range is between 3.0 Mbps - 25Mbps (which can be a minimum 7X higher than Dolby Digital)
Most DVD's have Dolby Digital while some have DTS
Blurays have Dolby Digital, PCM lossless and either DTS-MA or Dolby TruHD
Some players like the Sony PS3 (older version not the slim one) can only do PCM lossless audio and most people cannot tell the difference between PCM (Pulse Code Modulation) and bitstreamed DTS-MA and Dolby TruHD (I can BTW)
Speaker cable (don't believe the Monster BS) any standard 16 gauge will work just fine, 14 is better and 12 gauge will work for 99% of the population. The lower the number the thicker the wire strand to pass the audio signal.
Cables, again Monster is nice but way overpriced most require HDMI and there is not one difference between a Monster 150.00 HDMI and a 3.00 internet HDMI cable, all they are doing is passing a digital signal which cannot be manipulated by a cable in any way, shape or form. The cable either works or it doesnt and anyone who tells you different doesnt know what they are talking about. Analog cables are a different matter because they can be maniplauted with RF frequencies and interferrence, but most cables today are well shielded and not much of a concern.
I would read as much as you can, visit sites like this one, Sound and Vision Forums (read the pinned threads by a member called IrritateGuy -IG) or AVSForums where they have many good threads on various gear.
Good luck and congrats on the new job. Sorry so long guys.