i'm soo excited for you southampton kitty!!! i'm sure your trip to paris would be a blast!
based on my experience in paris just this june, i love the store at rue royale - off faubourg st. honore and just a couple of streets from rue cambon where the main chanel is located. fyi, there's a chanel just next to the rue royale dior too but that chanel is one which i'd NEVER visit again - (more than the mothership at ave. montaigne) - the top level of rue royale was where i swore i saw the book with the exotic leathers in it, plus all the beautiful clothes and out of this world bags - like the lizard & croc jeanne bag). avenue montaigne is somewhere which you must visit of course, since it's the mothership, there's even homeware in one of the rooms there, and dior cosmetics and room upon rooms of clothes!
and no worries about the SAs being snotty/poor service. i got some PRETTY BAD service at chanel (SA practically raised her voice at me), but overall, i got the best service at dior even though i didn't buy anything. i just LOVED being in the dior boutiques best and the inventory is mindblowingly good!
pricing wise, i'd say it's really good plus with it being tax-free, it's really quite dreamy. the SAs speak excellent english and i'd recommend Rie (I think she's japanese) at the rue royale store - real patient, very friendly and enthusiastic and she knows the bags well.
everyone's given you really good tips. sensible (but stylish) shoes would be good too since my feet were practically aching after just walking along champs elysee. dress to impress definitely and it may be just me, but i did notice that carrying a designer bag helps get you faster service than if you were to carry a non-designer bag. the dior in the department stores (printemps? or lafayette? my memory is going wonkers) isn't too impressive so i'd give it a miss if you're short of time.
it's pretty walk-able down from champs elysee to ave montaigne and then rue royale (a nice slow walk though) but if you're taking the metro, the nearest stop to the rue royale one would be the concorde, it's just a straight road down from the exit at concorde.
and you've got to try leon's along champs-elysee, i love the mussels there.