I've only got about 1 year of experience running a trail camera, but these items are what I consider important when choosing a camera... in no particular order:
2) Battery life
3) Size of the camera
4) Picture/video quality
6) PIR sensitivity
Now my opinion is going to be biased because this is the only camera I've ever run on a regular basis on my hunting grounds, but in my opinion the Bushnell Trophy Cam is the best 200 dollar camera on the market right now. When I worked for USDA, we ran nothing but Reconyx cams... and let me tell you, those cameras are amazing. And of course we've all seen the quality and performance of the BEC cams, thanks to members of this site and others. However, those cams are high in price and just aren't practical for MOST of the deer hunting community (again, just my opinion). My Bushnell only cost me 200 bucks, plus about 15 bucks for a couple memory cards, and 16 bucks for a set of batteries... so say 231 bucks total. I bought this cam in December of 2009, and to this date I have not put any more money into it. I'm still running the same set of AA batteries, even after over 7000 pictures and a couple hundred videos. Sure, you can find cheaper cameras, but if you're changing out batteries every month or two, what's the friggin point? The Bushnell is about as easy to use as it gets, the 8 mp picture resolution is amazing, it's tiny so people can't easily see it on a tree, and with 3 different PIR sensitivity settings this camera doesn't miss a thing that walks in front of it. If necessary, you can also buy a security box. In my opinion, this camera is by far the best bang for your buck. Especially for a guy like me, who doesn't live at the property he hunts... I make it down there every couple weeks if I'm lucky, and I know the whole time that my cam is out there doing its job, just like it should be.
When it comes to effectively patterning deer, due to lack of experience I really don't know how many cams it would take to cover 640 acres. I think the biggest determining factor is going to be the type of terrain that is on this 640 acres, though. I do know this though... the property I hunt in Carroll County is less than 100 acres total, and I wish I had at least 3 to 5 cameras on it. Multiple that by six, and you're looking at a shit ton of cameras to monitor one square mile.
Even though I'm not a seasoned veteran when it comes to running cameras, I think I've gotten pretty good at effectively setting them up. The way you position your camera will vary, depending on the type of the cam, its effective range, and whether you're capturing still images or videos. The time of year also plays a big role on where you place your camera. I like to set mine up relatively low on the tree, usually right around waist-height. I find that positioning it low results in well-centered pictures of the deer... which of course is the goal... good pictures equate to good scouting. I always pay attention to which way the camera is pointing. If at all possible, I try to position facing north or south so as not to get blinded by the sun... of course this isn't always feasible. When capturing still images, I'll set the cam up overlooking frequented trails and funnels, and I'll usually set it up to take 2 or 3 picture bursts. Video capturing is usually saved for scrapes or concentrated food sources, like a loaded apple tree.
For me, the bottom line is that I only have a limited amount of cash to spend on hunting and fishing related gear each year. This is why it took me so long to even purchase a trail camera. And when it came down to deciding which one to buy, I needed to get the most performance for the money. It's the same reason my wife and I are buying a new Chevy Cruze today instead of the Ford Fusion... my family all works at Ford, and the Fusion is a great car... however, I can get a fully-loaded Cruze for about 1500 bucks less than a base-package Fusion, AND still get better gas mileage with a turbocharged 4 banger. The Bushnell TC is a very affordable, yet extremely capable trail camera. It's done nothing but perform for me in this past year, and I couldn't be any happier with it. If I were to buy another camera today, without a doubt it would be another Bushnell.
Good luck with your article, Mike. I'm sure you'll get some great feedback here. Welcome to the site!