Assuming that IdahoHiker will be hiking in the Idaho mountains, he needs a bag that will take him at least a few degrees below freezing, even in summer. Of course, as many have pointed out, putting on insulated clothing inside the bag can help extend its range. So can a breathable bivy sack or (if it's below freezing) a vapor barrier.

Just remember that most 20* sleeping bags, except the really expensive kind like Western Mountaineering, are more like 30* bags. There is unfortunately no standard at all in the US for rating bags. There is an EU standard (rather complicated) which is a lot better. However, for those manufacturers who sell their bags in Europe, you may have to go to their UK site. And I've found that they seem to sell different models (or the same models with a different name) in Europe.

If the REI Sub-Kilo is still on sale, that's the bargain of the century--snap it up if you can get one! If not, the Campmor down bag I mentioned.

Pay attention to girth measurements--measure your own (preferably over your insulating jacket) and compare to what's given for the bag. You don't want a bag that is so narrow you're uncomfortable (or, worse yet, compressing the insulation), and you don't want one that is too big, giving you too much dead air space to warm with your body heat.

thecook has pretty well summarized the down vs. synthetic thing. A couple of notes: If properly cared for (don't store any bag compressed), even a lower-quality down bag lasts longer than synthetic, which doesn't recover as well from being squashed multiple times. (2) Despite manufacturers' hype, a soggy synthetic bag is no warmer than a soggy down bag (been there, done that). However, it's a bit easier to dry out and regain the loft with a synthetic bag. Wet down tends to clump. If you don't have a dryer and tennis balls (and you won't when out in the wilds), it's harder to get the loft back with down--you'll spend a lot of time pulling apart the clumps. Better yet, regardless of the insulation type, don't let the bag get wet! Check your camp site to be sure it won't turn into a lake in case of a cloudburst (once again, I've had bitter experience). Use a trash compactor bag pack liner (as mentioned in one of my earlier posts here) or use a mylar turkey roasting bag to protect your sleeping bag. Keep your shelter where you can access it first without opening up your pack, set it up, and unpack your pack under cover. If you're using a tarp instead of a tent or tarptent, consider a bivy sack (make sure the top fabric is breathable) to protect your sleeping bag from splash or wind-driven rain. Don't cover the bag with a non-breathable material, or the moisture in your body will condense on it and then get your bag wet. When it's below freezing, the moisture from your body (which is always there) is apt to freeze on the inside of the outer shell of your sleeping bag and will wet the insulation--this is where a vapor barrier (inside the bag, over your base layer) is a good idea.

My own perspective is to take synthetic insulating clothes and a down bag, so all my insulating eggs aren't in one basket, so to speak.

Re your clothes, it's great that you found so much in your drawers and closet--I should have said that's the first place to look--my bad! The "nylon cheapy pair of shorts" will do fine doubling as a swim suit. In the wilderness, nobody worries about "street clothes" in the "pool." If you take both shorts and long pants, don't bother with zip-off pants (the zippers, I've found out, can be a real nuisance). The main purpose of the zip-offs is so one item doubles as both long pants and shorts. If you want them, though, Campmor's "Trekmor" pants are $30. I did see some on sale last July at Big 5 Sporting Goods for about $22. I assume those athletic socks are synthetic. You could get a pair wet and wring them out by hand to see if they're relatively quick drying. If you do the same with a pair of cotton socks, you'll have a good comparison. Forget the fleece pants for summer hiking. Thin baselayer bottoms will do fine. Lightweight is best at wicking moisture from the skin, which is what you want. Unfortunately this is not a good time to find lightweight baselayers (formerly known as long underwear) on sale, but look around. Watch Campmor because even when their stuff isn't on sale, it's often cheaper. The Duofold polypropylene baselayers are only $10.99, but they only have midweight right now.

The poncho should be fine, but I'd suggest either knee-length gaiters or silnylon rain chaps with it. Hiking through wet brush or grass after a rain or even a heavy dew can get you wetter than walking in the rain. I haven't hiked in Idaho, only Colorado and Wyoming, but I'm assuming you also don't have the waist-high brush we have out here in the Cascades, which pretty much requires full rain pants to slog through. I know that where I've been in the Rockies, knee-high gaiters are plenty long enough. You also want a wind shirt--find a cheap unlined nylon jacket and put DWR treatment on it--to keep your arms a bit dryer. (Those should be available in the spring, if you can't find a thrift store model.) (If all you can find is a nylon jacket with a flannel lining, cut out the lining.) The wind shirt also helps to keep bugs off you in camp and at rest stops and, of course, repels the wind. Lots of times the wind shirt is the only wrap you need, especially when you're on the move. If it's just drizzling a little, the wind shirt, which is more breathable, may be enough to keep you dry. Also, look for a cord or something simiiar to belt the poncho snugly to you when it's windy. The poncho also protects your pack (although, like a pack cover, it won't help if you fall in a creek). It sounds as though you're just about there on clothing--congratulations!

Just a warning--in return for all this free advice, we expect to see some trip reports next summer about backpacking in the Idaho mountains (or wherever you go)! Please let us know how your gear works out!