I certainly don't mind answering this question again, but just exactly how specific does my "mission" have to be?
As specific as it is!
Some people only do one type of flying. For example, if you want to go by yourself really fast for longer distances and you're tall, you want a Mooney. If you want to fly around the local area from a 1000' grass strip by yourself, maybe a Super Cub. If you want to load up your wife and all four kids and stuff for a week on a regular basis, you probably want an Aztec.
However, there are "wide-mission" planes too, and Cessna does a great job building that type of airplane. If you want something that's cheap to operate but will do a variety of things, a 172 is hard to beat. The 182 will be a tad more expensive to operate, but it'll do darn near anything you ask of it except carry more than four people or go faster than 140 knots in level flight.
It sounds like you're a "wide-mission" kind of guy, and that you like to do a lot of different types of flights, but that you do a fair amount of long cross-country flying, mostly with you and your wife. It also sounds like you'd like to go somewhat faster than your 172 goes, so that you can get places in the same amount of time. You want something that's easy to get into, and comfortable inside for the longer flights. Am I right?
So far, this says (to me) 182 or 182RG all over it - The extra cabin width will keep you comfortable and allow you some biscuits too
, the extra power and baggage space means you can take more stuff (a pair of bikes, maybe?) if you want or even a couple of friends/kids/whoever. The FG vs. RG question is one only you can answer: Is the acquisition cost and extra maintenance of the RG worth it? An older 182 will do 130-135 KTAS, a new one will do 140 KTAS, both on about 13 gallons per hour. The RG's will do about 150.
As far as speed goes, yes you can enjoy a flight, but a 172 is slow enough that you can get a pretty good percentage faster without spending a fortune. The 182RG will cost you maybe 50% more to operate, but you'll be getting 40% faster airspeeds as well. Trying to get another 40% faster than the 182RG will cost you an additional 100%-150% (over the 182RG). I've done Houston to Madison in a day and Denver to Madison in a day in a straight-leg 182, and I've flown it to the east coast, west coast, and gulf coast from the Midwest (see my map below - The vast majority of that was in the 182).
No matter what you do, before you spend any money on upgrades, be SURE that you're in the right plane. It doesn't sound to me like you're sure of that at all, and it does sound like you want a plane that is somewhat faster without sacrificing the ease of flying, versatility, and comfort you get from the Cessna. Once you've got the right airplane, upgrade away!