Your choices at this point are:
FCP7
Great software, great interface, solid, need to transcode all incoming footage, discontinued.
FCPX
Powerful foundation, will edit H264/AVC HDSLR footage in real time, very buggy, clunky user interface.
Premiere
Interface not as good as FCP7, better than FCPX, if you have the right graphic card it’s more powerful then FCP7.
Avid MC
Expensive, limited, reports are that it works well, though I haven’t heard any reports from iMac.