the shared memory is if you are using the built in gpu of the cpu. since you are using a separate gpu card, then this does not apply.
as for how much room the case has for a gpu, there appears to be a lot of room for a card. i'd expect you'll have no problem fitting a 750ti min there. but to be sure, you'll have to measure it when you get the machine in. the pictures don't really tell the full story and i can't see any case model listed on the pictures to look up. looks like a good bit of room but that depends on the size of the card in it now. no idea what that is so can't guess on how much more room is there.