The guns and ammo are probably a lot more fun to acquire (at least for me), but, realistically, once you've performed some minimum "due diligence" in those areas the other stuff becomes a whole lot more important if you're really worried about a survival situation.
You need shelter. Hopefully you can stay at home. If you're in an inner-city high-rise, that may not be a good option. Where are you going to go? Do you at least have a good tent? Or do you just plan to shoot some remote farmer and take over his house; if that's your plan I hope he gets you first. I'm serious about this. If you have to leave home, there will probably be, depending where you live, tens or hundreds of thousands of people doing the same thing, all of whom will run out of gas within a 200 mile radius, depending how full their gas tank is. How full is your tank? Got any extra gas tanks in the garage? Where are you going to go, and how are you going to get there? And if you live in Bigtown USA, where there's a smallpox breakout, you'd better not expect those friendly folks in Smallville USA to welcome you with open arms. You'll probably be on your own, at least for a while.
Real soon, you'll need water. Hopefully, you've got at least a few days worth in the basement. Once that's gone, your PUR Hiker is only going to cut if for about 100 gallons, and your little bottle of iodine treatment might add about 4 gallons. My suggestion is the Katadyn Gravityn [sp?], which is good for 30,000 gallons, plus a replacement filter or two. Costs around $250, without the extra filter, but that's going to be more important than having your third AR.
If it gets cold where you live (e.g., it's about -2 degrees outside here now), you need heat. Do you have a wood burning stove? How much wood? Chainsaw? Fuel? Extra chains? And do you have good sleeping bags, and I'm not talking about the Coleman 40-degree bags at Wal-Mart.
Gees, we haven't even talked about food yet. MREs are fine for a few days, but otherwise they're way too expensive for most of us. You need bulk quantities of staples: flour, salt, sugar, baking powder and soda, rice, oils and fats, wheat, beans, etc. Stock up on some multi-vitamins. And if you make it through the first year, what will you do in Year 2? You need seeds for vegetables; start a garden, there are freeze-dried vegetable seeds all over the Web. Also, you'll want some fishing gear to help supplement the diet, and maybe some traps or snares, and the knowledge to use them.
Sooner or later, someone you care about will get sick. It would sure be nice to have some broad-spectrum antibiotics. Actually, you can pick them up at farming/animal supply houses (I'm not a doctor, I'm not representing anything, but I understand that some of them, like Tetracycline, are chemically identical to the human versions), but they're no good if you don't know how to use them. Do some research, find out what works, buy some stuff, and print the dosages on the container (so if you're dead someone else can figure out what to do). Plus, you may require special medications. If that's the case, explain your concerns to your doctor and try to get a three-month stash. Hopefully, things will blow over by then. In addition, you'll want to have a first-class first aid kit, which is a subject in-and-of itself.
You will want light. The MagLite will not cut it, at least after a couple of hours. I suggest LED lights because they last so much longer; the LEDs last virtually forever, and battery lives are extended by a factor of 10 or 20. Some rechargeable batteries and a small solar battery charger would probably do the trick.
Sanitation will be critical. How are you set for soap? Do you know how to make soap from ashes and animal fat? How much bleach do you have (you can more-or-less purify water with 8 drops of bleach per gallon; double the dosage if the water is cloudy)? Toilet paper, toothbrushes, toothpaste, etc.?
You'll want a radio (the more bands the better) to find out what's going on, and batteries to keep it going. Baygen Freeplay (the name may have changed) makes a very good one that can be powered by a hand crank or solar energy.
Since you want to maintain a will to survive, some "fun stuff' is essential. Simple games (checkers, chess, cards, a frisbee, whatever) and some treats like hard candies.
Sewing stuff.
A couple of survival books. Books that will tell you what plants you can eat, how long to boil water to sterilize it (5 minutes should probably do), how to preserve preserve (salt or smoke or dry) the food you obtain, medications used by native americans, etc.
Set aside some cash, nothing bigger than $20's. As much as you can comfortably afford. Some gold or silver coins would also be a good idea. Many people recommend taking copies of your important documents, too.
Miscellaneous stuff like a compass, tools, knives, multitools, fire starting gear, rain gear, maps, shovels, etc., etc.
When all is said and done, the most important thing is your brain. What do you know how to do, and what has your family learned how to do.
Gees, I just went back and read this, and I can't imagine why I was inspired to write such a long and marginally off-topic response. I guess it's just the fact that instead of spending $600 on yet-one-more gun (or ammo), there are a whole lot more practical and helpful things we can do to protect our families in case of a SHTF scenario. Sorry for the diversion.