Regarding 1 Tim 2:9 & 1 Pet 3:3, is this a command from God, that women do not try to look pretty? Or, is this a suggestion from Paul? How serious should I take these verses?
This is a great question, and I appreciate your concern to do God's will. There are three basic ways to handle such texts as these. First, the apostles may be making a comparison that is not intended to be absolute. That is, they are saying "If you have to choose between outward beauty and inner beauty, choose inner." They may well express such sentiment in absolute terms, but this is probably in emulation of how the Lord Jesus often taught. He often used hyperbole to get his point across.
When, for example, he says, "I desire mercy and not sacrifice" (Matthew 9:13; 12:7), the point is not either/or but one of relative value of each.
In both of these passages, the Lord quotes from Hosea 6:6, where God speaks to the nation and tells them that their sacrificial system has become corrupt. "Mercy" is the heart of the whole thing, for the sacrifices point to God's mercy. If this is what Peter and Paul mean in these texts, then they are simply giving relative worth to inner vs. outer beauty. In the least, this is what the text must mean. But it may mean more.
Second, it is possible to read these texts as completely culturally conditioned, so that their relevance for us today is nil. That is, it is possible to read them as absolute prohibitions but which no longer have validity for us today. The reason for such would be that often the New Testament gives instructions that are meant to be valid on a literal level in their day, but are no longer such today. "Greet one another with a holy kiss" (Romans 16:16, etc.) is one such text. That was a cultural notion that doesn't play well in America. An apprpropriately similar gesture is a handshake or, in certain contexts, a hug. The keys in determining whether such things are culturally conditioned are complex, but suffice it to say that if an author does not ground his commands in a larger reality (such as Paul's prohibition of women teaching men, a point he grounds in the creation account), then commands that address just externals may well be culturally conditioned. In the end, I think a very important principle to follow is this: If in doubt, always follow grace.
Third, there is another way to read these texts. It is possible to read the braiding of hair, the gold clothing, etc., as something that the very, very rich would put on. That is indeed likely. Those in the first century who wore such clothes were usually the extremely wealthy, and they would flaunt their wealth in front of other Christians, most of whom were poor and many--if not most--were even slaves. Especially in the 40s, when James wrote his letter (notice chapter 2 that speaks to the same kind of clothing), there was a famine in the Mediterranean world. There was deep poverty that suddenly struck the known empire. Those Christians who paraded their wealth when others were starving were acting in a very ungodly way. This view would thus have a culturally-conditioned element but also some universal application. How should we apply it today? In many churches, to wear rather expensive clothing, or to drive expensive automobiles, would be to flaunt one's wealth when others are hurting. That's not appropriate behavior. In other churches there is great wealth among the members, and this kind of thing would not be inappropriate. I grew up in Newport Beach, California. I went to a great church which had a lot of insanely wealthy people in it. It never troubled me to see them drive to church in their Ferraris, or to see them wear furs or expensive clothes. I just hope it didn't trouble them to see me wear blue jeans!
The point is that none of these views are prohibiting a woman from trying to look her best, as long as it is within reason. That is, the flaunting of wealth seems to be the major focus here. At the same time, even a woman who does not flaunt wealth but focuses on her outward beauty to the neglect of her inner beauty is in violation of these commands. My advice would be: as long as you are first making sure that you are beautiful on the inside, go ahead and put on make-up, wear nice clothes, look pretty. As far as I know, only the most extreme legalists within the church would argue against this kind of advice. And remember: the overarching priniciple is this: grace triumphs over all.