The Viewsonic has DVI (digital-white socket) and VGA (analog-blue socket). I'm not familiar with the iMac or Display Port so I can't comment on compatabilty there, but I did do a fast Google search and found that the Viewsonic site states that the monitor IS compatable with the Apple PowerMac G3/G4/G5 models. The other reviewer (Mr. Massetti), sounds more knowledgable about this one. Sorry I couldn't be more definite.
Yes it will work on iMac there Two Thunderbolt ports Mini DisplayPort output with support for DVI, VGA, and dual-link DVI (the adapters are sold separately) you just have to purchase the right adapter that you want to use either VGA or DVI, they cost $29 each.