My band is looking to buy a PA (at last) and we're having a disagreement over how loud it would have to be and how much we'd have to pay.
could someone tell me what sort of wattage we'd need and how much to spend on a first PA system.

P.s sorry if this is in the wrong place
Will you run just vocals through it or an entire band?

Do you just need mains, or do you need monitors?

Will it be for rehearsals or gigging or both?

What size room and audience size are you looking at?

Without these answers, nobody can suggest anything to you.

Could I get some more talent in the monitors, please?

I know it sounds crazy, but try to learn to inhale your voice. www.thebelcantotechnique.com

Chris is the king of relating music things to other objects in real life.
^^^ The man is right. But I will answer some of the questions.
Also most gigs you would have in house sound, at least around here. So all you need to bring is guitars, bass's, drums, amps, etc.

If your running the whole band through the PA, your going to need to spend a lot more money. If your running just vocals (lead and backing) you won't need much for the PA.

Now I bought my PA system knowing that one day in the near future(hopefully) I will have gigs. So I built the PA system so it'd be capable of doing a gig. I still have to add subs and more mics, compressors, and EQ's but im not in a band yet. And I won't buy that stuff unless we NEED it. Like I said above, most places around here have in house sound. My set up is already for practice with the exception of adding more mics. So your goal here makes a huge difference.

Lets assume you just need a PA for VOX only and for practice.

You need:
A mixer
Two mains
Speaker and mic stands
Mic's (SM58)
Monitors. In ear or floor monitors. I recommend in ear. They are cheaper and better in the long run. They do not require an amp like floor monitors unless your get powered ones but that's more money. And another plus, there won't be a lot of bitching about the lead guitar being to loud etc.
Compressors(take some time to really learn how to use them. They will dramatically help your vox or dramatically hurt them)
31 band EQ. 31 band only! 16 band is pretty much useless compared to a 32. (geared toward vocals)
Power conditioner. Get one that has plenty of outlets. Power conditioners are important. They are much more than a surge protector. You don't want your $2k PA system to fry or go up in smoke because of electrical surges or whatever. I myself have to buy one.

Now if you might be putting the band through it or gigging. Everything above is needed you just have to add two subs and another amp for the subs.

Now as for what brands to look for. I wanted the best. I didn't want to go cheap at all. Music is my thing. It's the only thing I want to go all out for.
Look at Crown for amps. Peavey or JBL for mains and subs. DBX for EQ and compressors. Now that is highly biased. Just like I rather use K&N air filters instead of injen. Hah.

Any questions. AIM me. HoviPB87
-PV14 Mixer
-Crown XTI 1000 Amp
-Two PR15 mains
-Shure SM58
-Dbx compressor/gate