If you own a iMac, reason 6/ logic 9, a midi keyboard and a pair of headphones, what would you buy next? (1000 in your piggy bank)
If it's £1000 British quid I'd go for an Apogee Duet 2 and a pair of Adam A7x monitors.
If it's $1000 dollars [US, Aus, Can, NZ] then I'd go for a nice colourful woolly hat and hope I had enough for a taxi home.
Headphones will only get you so far. The most important part of the signal path are your D-A converters [your soundcard] and your speakers.
An internal iMac soundcard and headphones will show their deficiencies quite soon.