Well, never been to Paris but I’ve studied French for the last four years…lots of Parisian study within that, so maybe I can toss in some ideas.
 
First off, is Paris just a stop, or is the whole trip going to be spent there?  If you’ve got a week to vacation you’d probably be wise to pick two cities and really relax and enjoy them, instead of spending too much time train-hopping.  If it’s a month-long trip or something, you’ve got more flexibility anyway and if you’re liking Paris a whole lot, no reason not to stay longer!
 
I’d say 3-4 days would be pretty good.  You could spend ages in the first 8 arrondissements alone (includes Eiffel Tower, l’Arc de Triomphe, Champs-Elysees, Place de la Concorde, Louvre, Ile de la Cite, Quartier Latin), and if you feel like a daytrip to Versailles the C5 train runs right through there with a bunch of stops.  As far as hotels, depends on your budget!  Haha…if you’re looking to splurge, Grand Hotel Jeanne l’Arc…if you’re looking to save, there are boatloads of hostel options.  Check out a Let’s Go: France guide, it’ll have tons of info for all budgets.

We used “lonely planet” to help us plan trip to Paris, and wouldn’t recommend it.  Our favorite series is Rick Steves, and he always has authentic, reasonably priced lodging suggestions.
 
We really liked the Louvre  (check out the suggestions in the guidebooks about the best time of day/week to go, to avoid lines).  We also enjoyed the famous Ferris wheel ride, which gives you a “birds eye view”.  Notre Dame is a must see.
 
If you decide on a side trip to Versailles, go FIRST THING in the morning.  The lines can be CRAZY.
 
 

Your first step might be to get one or two good guidebooks to Paris, read them, and think about your own “Must sees”. My own preference is the Michelin Green “Paris”, but you might prefer another.
As you form ideas about what you’d like to see, you’ll need to understand here things are within Paris, and how you’ll get there, so be sure that you get familiar with a good map of Paris and the metro/RER system, so that you can get from your hotel to places you’d like to see.
On your arrival day, you might be so tired from the transatlantic trip that you’ll really see nothing, so count arrival day as a zero, and plan to spend a minimum of 3 days, and preferably 4 to see the things that you’ve chosen to see. Keep in mind that Paris is a great city to just walk in, so some things that you’ll like might be just a short walk from your hotel.
I don’t think you should look for a guided tour—- it’s an expense that you don’t need, especially if you do your own “homework” before you leave.
As far as hotels, the dollar is very weak right now, so be prepared for high prices! You can, however, find hotels that are fairly reasonable ( I mean prices similar to those you’d pay in major American cities). There are lots of search engines that you can use. Lately, I’ve been staying in Ibis Hotels for price/convenience, and you might want to look at their website just to get an idea about prices: Accor.com has hotels ranging from luxury (Sofitel) to Novotel to Mercure to Ibis. Just be sure that any hotel that you ultimately choose is within central Paris (Within the Peripherique, which is a highway encircling Paris; outside is cheaper, but usually means long commutes, fewer metro staions, and potential problems if you stay out late).
Finally, I always urge people to try a short trip outside Paris to a small town, and there are dozens of possible choices! The TGV (=fast train) can take you to many destinations in only 3 hrs even, for example, to Marseille. It’s even possible to take a TGV from many places non-stop to CDG Airport. Here’s an example: Lyon to CDG in about 2 hrs (central Paris to CDG might take one hour!).

Don’t forget a day or two if you wanna stock up on some shit: clothes, music, books, wine, etc… 

are you going anywhere else?

i spent 5 days in Paris and could have stayed longer

say hello to Napoleon for me

Kameljit,

Where are you coming from?  And what’s the maximum amount of days that you can take for this vacation?

Is there any reason why you must spend your entire time in Paris?  Basie made a great suggestion: to get out of Paris for a few days.  Plenty of places around Northern France, and thanks to the TGV bullet trains the South of France is well within reach (and highly worth it in September).

Do you think you can do 8 days?  Spend, maybe, 4 days in Paris, and another 4 maybe in Arles, with perhaps a 1-day or 2-day excursion from Arles down to the Côte d’Azur either by train or car rental?

I mean especially if you’re coming from North America and you’re going to fork out all that money for a long flight, might as well maximize your time in the country, and additionally, you’ll get tired of Paris after a few days, and you’ll want to go someplace new.  September is smack in the middle of wine harvest region, you can visit one of France’s many wine producing regions.

 
Don’t drive in Paris if you want your vacation to be a memorable one

My wife and I were in Paris as part of our Europe Trip last year. We were in Paris from Sept 22 – 27th, thats about 5 full days in total. That amount of time was good for us. We went to the touristy spots as well has had some time to relax and just take everything in. The betterpart of our days were spent taking in sights, then we would wind down in the evening with a nice meal, some wine and the days just flew by:
 
We managed to see:
 
– Eiffel Tower – set aside 4-5 hours if you want to go to the top
– The Louvre – 1 solid day – we left the Louvre around 4:00 PM and just walked along the Seine River for a few house, that was nice as well
– Musee d’Orsay
– Natre-Dame
– Arc de Triomphe
– Cimetiere du Pere Lachaise – Jim Morrisons gave
– Invalides
– Basilique du Sacre-Coeur
 
We also did a Paris Illumination tour one night, it cost ($40 or so each) and it was great, we saw Moulin Rouge all lit up
 
We also had 1 full daytrip where we went to Normandy and Mount St Micheal.
 
5 days was good for us, we were busy, but that was the way we liked it.
 
Good luck
B