1. Bring your own phone if you can (unlock it first). If you can't, it's not a big deal.
2. Buy a SIM card (or phone) at Bouyges, and use their prepaid card, La Carte Nomad. It gives you twice as many minutes as the regular Bouyges prepaid cards and Orange's cards. The catch is that they erase your minutes twice as fast, but you can avoid this buy recharging your minutes before time runs out.
3. Definitely not necessary to have a land-line if there isn't one already set up. If there is, it might be worth checking out, but having a cell phone is definitely useful, and texting is crucial since it's much cheaper than calling.
Long version:I've had two experiences with getting a phone in France. The first time, I brought my own phone and I still had to buy a SIM card, which I think was about 15-20 euros. In order to do this 1. you have to make sure your phone is compatible with their carriers and 2. call your phone company to get a release code, enabling your phone to still function if you put in a different SIM card. For my second time, the code wouldn't seem to work, so I had to get a phone. I think the cheapest ones are about 40 euros or so, and they come with a SIM card, so it's not really that much more expensive if can't use your american phone. The program runs these informational sessions, and one of them is with a phone store called The Phone House, and they will talk to you about what types of things are available. There are a lot of stores like this one all over town, and you can buy a phone at any of them - they're all pretty much the same. You can take a contract, but you can only do this for a minimum of one year, and there's a lot of forms to fill out, and I think you need to give them bank information (which you have for a couple weeks after you get there), so it is easier and quicker to buy prepaid cards. You can buy the cards at most dealers of your carrier, and even at the ED (the supermarket) I tink they sell them. The two main carriers are Bouyges (pronounced boig) and Orange... who both offer prepaid cards for a standard flat rate, 10 centimes a text message, and 50 centimes a minute to talk (I know, it's expensive, but keep in mind that you don't pay when someone calls you). Additionally, they do erase your minutes if you do not use them during a certain duration of time (that depends on which card you buy), but if you buy a new card before time expires, your old minutes will accumulate and will not be wasted. However, I found that Bouyges had an option for a prepaid card (called the La Carte Nomad) that charges you half that per minute (25 centimes), and texts are still the same, but the catch is that they erase your minutes in half the amount of time (but you get twice as many minutes). I think it ran for 25 euros, it lasted four weeks, and gave me 80 minutes or its equivalent in SMSs. I found it normally lasted me about 4 weeks and it wasn't too expensive, so I'd recommend that.
That's quite a lot of information for such a seemingly little question, but it can be really confusing, so I hope this'll help.