If I remember correctly, my 750's hold around 698 after formatting.
EDIT:
Originally posted by: jtvang125
Also, isn't it still more economical to go with 2 500gb than 1 750gb?
Money-wise: yes
Space-wise: depends
For me, I was an idiot and bought the smaller version of my main system's case (Lian-Li PC V1200B-plus) and thought it'd be fine size-wise. Well, I was wrong so I've got limited space for drives. I could technically go for 3x 500 rather than 3x 750, but I want the extra room too.
Oh and according to windows calculator, 698.49193096160888671875, is the size. All you do is multiply the listed gigs by 1,000,000,000 and divide by 1,073,741,824 (1024^3).