Just curious, if I were to get a premium membership and want to upgrade to a corporate membership, would I need to pay the full amount of the corporate?
Also, does the corporate membership come with the same benefits the premium membership does?

Just curious, if I were to get a premium membership and want to upgrade to a corporate membership, would I need to pay the full amount of the corporate?
Yes, you would have to pay the full amount.
Also, does the corporate membership come with the same benefits the premium membership does?
Nope, it is treated as a separate membership.

Then are there separate corporate member forums?
Also, can you have a membership in both, corporate and premium?
It'd be sort of cool to have both of the little user bars. :)

Corporate Members have access to the Premium Member forums. There are no Corporate Member forums.
Premium Membership is explained here (http://www.webhostingtalk.com/memberships/premium/). Corporate Membership is explained http://www.webhostingtalk.com/memberships/corporate/.
I wouldn't see a problem with purchasing both.