Bit confused at the mo

I am just about to start my MCSA studies and decided to pick "70-620 Windows Vista Configuration MCTS" as an elective exam. I have already got the MCDST certification and did notice that there is an upgrade using 70-621 but is unavailable as an elective exam.
My confusion is:
1) Will the 70-620 still be an equivilant upgrade for MCDST?
2) Is it worth doing this exam?
Cheers