Robert S. said:
You don't need a PA as much as a monitor section for rehearsals. PAs are generally broken into 2 sections, monitors and front of house (FOH). Most of the stuff that comes in packages (PA head, speakers, ect) are designed for FOH use and are made to shoot the sound out to the audience. You can turn them around but the results are not always optimal. Basically you are taking a system made to throw sound a good distance and putting it in your face.
For rehearsals I use a Mackie board (get what you can afford), 4 Yamaha S112M monitors and a Crown CE 1000 power amp. By the time it is all wired up it does come in at about $3k.
Here's the tricky part. You need to decide if you want the capability to grow the thing into a full PA further down the road. Powered PA heads don't usually have a lot of flexability (or power) and that flexability is what you will need to run both a monitor andf FOH rig simultaniously. Once you buy a PA head, thats it with no room for growth and they can be hard to sell.
I'd suggest a board and even a Berringer is a start. They aren't the most rugged or toneful boards on the planet but they work and you can expand yor rig using one. Get as many channels as you can afford easily, the more the better. Then get a couple of powered monitors and a couple of mics. That will get you started and you can expand your rig from there. It might cost a little more than $800, but down the road you won't have to sell off your limitations to expand.
I've seen too many bands play through FOH rigs with no monitor section and they always wonder why they can't hear themselves. If you are primarily rehearsing, get a monitor section and get the FOH stuff later.