Mike,
Oo la la quelle bonne idée
I love Paris ANY time of year (but I AM biased !!) -August is always a good month because the Parisians leave 'en masse' and head down here ! However, as I say any time ....
DO NOT think of Driving !!
My brother has been over to see us several times by train, and the first part of the journey is, naturally, London to Paris. The ticket office/check in is on the lower level at St.Pancras. I'm sure if you book early you will get a 'good price' . You'll arrive at the Gare du Nord, Paris (2 and a 1/4 hours) . Have you given any thought to a 'package' ? It may be worth a 'google' !
I suggest you have a look at a good map of Central Paris - also a map of the RER (overground metro) and a Metro Map -there are lots on-line and some you can download as a PDF. You can then get your bearings of where the Gare du Nord is (10th arrondissement on the Right Bank) distance from centre etc. Although, like a lot cities the best way to see it is on foot. This site might help with accommodation
http://www.eurocheapo.com/paris/Things not to miss ... Notre Dame - the South Rose Window. Musée (museum) d'Orsay which was originally a railway station, (a lot of people don't see this preferring to visit the Louvre) The Louvre ( the glass pyramid is worth seeing if no time to visit the gallery, and the Mona Lisa) the walk from the Louvre through the beautiful (Les Tuileries) designed by Le Notre - Tuileries Gardens to the Place du Concorde where you'll see the Obelisk (from Luxor and over 3000 years old). There is a plaque to mark the place where King Louis XVI was executed in 1793. From there you can then walk up the Champs Elysées to the Arc du Triomphe where TEN lanes of traffic meet at the roundabout !! Montartre and the Sacré-Cœur - you can spend a good part of the day here, lots of artists and good eating places - don't go for the tourist menus !!! down to the Pigalle and the Moulin Rouge (but not at night !!). Galleries Lafayette (huge department store on the boulevard Haussmann) do not miss the beautiful Cupola entrance ..wonderful - )
I could go on but there's quite a lot for you to consider. If you want any more help - just ask. The things above are all the things I've done several times, plus many others. I want to go again now !!
Kate