I think Berkeley weighed OAT scores and GPA the same (if I remember correctly from reading their website). Some schools weighed them differently. Contact schools or read their FAQ for more answers to your questions.
340+ is my subjective opinion on a good target score for anyone with 3.2 gpa or higher. I also think getting a 340 should not be too challenging (or challenging at all) for anyone with a 3.2 gpa or higher.
If you are setting your crosshairs on the more competitive programs, you're going to want to get the best scores you can because your peers will certainly get them. But 340 scores will probably be enough to get you into optometry school in general.
However, low gpa and high OAT scores or vice versa will raise red flags and you will get the daunting question as to why you think that is, and that can be a tough one to answer.
To add to what mathcod said. I've seen more people get in with decent gpa and low oat rather than low gpa and high oat. I think in general schools weigh gpa more than oat but of course...anything uber low is a red flag.