I don't really like the 'cress' sound and even though I love Cassandra, I think Cressida sounds too self aware. In real life I'd probably feel differently though.
I'd want to pair it with something more simple sounding and friendly:
Cressida Amy
Cressida Rachel
Cressida Fay
Cressida Claire
Cressida May
Cressida June
Cressida Laura
Cressida Joanna
Cressida Joy
Cressida Blythe
Cressida Louise