I have three different PAs and I have found that I have had to revisit the issue with each one. I have a happy face on one, almost flat on another, and sort of a sideways S on the third. One good way to start is to get an experienced sound guy from a really good band in your area to help you out with a good starting point by standing in front of your system and pointing up and down as you scroll through the freqs. Make sure you are playing a song he knows really well!
He/she will probably do it for free since he'll most likely be flattered that you asked and it won't take long. Keep in mind that an empty room and a full room are often very different. You may have to boost the highs if you get packed.
Also, if you are buying a crossover, note that thet can have a HUGE impact on the overall quality of your sound. I didn't realize this until I switched from a Rane to a dbx Driverack PA crossover. It was like night and day.
And don't skimp on your mic. I recommend a Shure SM58 or Beta 58 for men and a Beta 87 for females or if you sing like the guy from Journey.