hello!
my 2 cents:
all in all you have 2 options if you wanna run a mac system (which is my preferable system nowadays)
if you go with the imac i would really wait til the new imacs will be out..
there is a rumor that those new imacs will be released soon and will have a "retina display"

i have a new macbook pro with the retina display and it is great!! and a big upgrade

all in all imacs are great computers, like the most apple products nowadays, although i have to say that my macbook is too slow and too small for editing raw video. (mostly because of the graphics card)
another option would be to build up a hackintosh. you can build up your own macpro with the fastest components available and will be only a little bit more expensive than the imac (but much better equipped) (i have build up one for video editing which runs a 4gb geforce 680, 32gb ram, an ssd for system files and a 6tb raid system for video files (4x3gb mirrored).
it works very fast and in realtime with raw files.
check tonymacx86 for more information..
the ony downside is that you have to be very careful with system updates.. (luckily i had no problem with my system so far..)
greets. sw