Fitzwilliam is my absolute favourite boys' name, and I also love Darcy. For girls, Elizabeth and Georgiana are both on my favourites list.
Just for fun, my favourite combos:
Fitzwilliam Deveraux (long, yes, but I love it)
Catherine Liberty Hazel
In the middle:
Felicity Catherine Amabel
Rosalind Victoria Jane/Rosamund Victoria Jane