Reply to 21" iMac & Dell Dual Screen problems with resolution

I have been running a i7 2011 iMac for a while now and decided i want to dual screen to help with my photo editing and music production.

After a while searching i decided to buy a Dell Ultrasharp IPS (Model: U2212HM). I have been able to connect it via VGA but was having problems with DVI..

So after leaving it connected to VGA i decided i wanted to up the resolution to 1920x1080. The manual says insert the CD for instructions on how to.. After googling it and searching for a few hours i still can't figure out how to..

Anybody have any input?

Much appreciated,