Why not use your WHM to upgrade the site? If your server comes with 2000 gigs of transfer then if that same server had 2 sites on it, I would allocate 1000 gigs to each site, or, 1500 to one and 500 to the other based on each site's individual needs making sure you do not over-allocate what your dedicated server comes with so that you do not get billed for transfer overages.