Bottom line: The 175ish length will gain you off-piste stability; but it won't be as good on the groomers as the 168's. The 168 will still be a way better deep snow ski than your old straight skis.
It all depends on what you are doing. Figure out where you are skiing-if you make 20 runs a day, are 10 off the backside, and 10 on groomers? Or is more like 16 on groomers and 4 in deeper snow? Be honest with yourself-I know plenty of people who ski say they ski 50%on/50%off, but when I ski with them, we MIGHT do 10% of the runs on the backside. I personally think of myself as a somewhat compentent all-mountain skier, but do I need a 50/50 ski? No way-I am mostly on the groomers, as we just don't get deep snow too much. Probably the biggest mistake I see people making is thinking that they need a midfat (75mm waist) for the 10% of the time they spend in deep snow. The rest of the time their ski is mediocre-they would have so much more fun on a shorter carver!
Think about your last season-how many runs approximately did you ski? How many were on groomers, and how many were in deep snow? In general, longer is better for deeper snow versatility, shorter is more fun and a better choice on the groomed. I ski both a 168 and a 175, but for different purposes. Any ski, any length, is going to be some sort of compromise. A longer ski won't be as fun of a carver. A shoter ski will lose stability in crud. A stiffer ski is harder to negotiate in deep snow and bumps, a softer ski not as stable or good edge hold on hard snow. Figure out where you ski, how you ski, and what you expect out of a ski. If you can't narrow it down, and you are super-serious or cash-laden, buy a quiver. Keep in mind that you are coming from 13 year old skis. Basically any ski you buy will be light-years ahead of what you were on previously, IN EVERY CONDITION! Better stability, carving, float in pow, crudbusting, bumps, you name it.
FWIW, you could go with the 168, or you could go up one size (probably around 175, whatever is offered). I would go shorter if it were me, unless I were spending ALOT of time in deep snow (talking 10 runs or more per day). My 168's perform very well in all conditions-I really only break out the longer 175's for those deep snow days. I have skied alot of those cross-style skis in both the 168ish and 175ish length-I just feel the shorter length is a better choice for most people. If I wasn't hooked up with pro-form pricing, I wouldn't own the 175 midfats, as the 12+ fresh days that they shine in are too rare. BTW, I was talking with a guy today who was demoing a pair of Fischer RX6's in 175-he looked to be about 6 foot 3 and a big guy. He was coming of a pair of 205 straight skis. He implied he would be going down to the shop and buying a pair that afternoon.....