Looking at the close-up photo, I just noticed the dates below the nameplate. It's tough to make out, but zooming in on the original high-res version of the photo, I believe it says she was born in St Petersburg in 1807 and died in Paris in 1845. I'm really stumped as to who's in this tomb and what their story is. The names certainly make it sound like there are two people, but the dates are clearly for just one person: the first one says "born" and the second says "deceased." Hooboy, I really don't know enough French/Russian history to be able to sort this out, and I'm certain as soon as I post this some brilliant reader will clue me in. Anyway, I like the tomb! It's a nice tomb.
Update: Check out the comments below which clear up some of my confusion! Comtesse Marie Potocka and Princess Soltikoff were indeed one and the same. Thanks for your help, readers! I wish I could tell you exactly where this tomb is situated in the cemetery, but I can only tell you it's toward the North-middle or North-West section.