Probably not the perfect answer from my side, but could be helpful nonetheless.
I'm using Cubase as my DAW and it has a nice Freeze function to reduce the required processing power and RAM. You tracked your virtual instrument and you're happy with it for now ... then just freeze the track and save the processing power on this virtual instrument until you unfreeze it to make some modifications. Very nice and helpful feature for advanced projects. Not sure if other DAWs have similar option(s) though. It's like a temporary bounce.
So if you have such a feature, then you shouldn't run into problems, even with a slower hard disc. Actually, the transfer speed of the disc isn't the limiting factor, even on a 5400rpm disc. It's more about the file access times provided by this physical discs.
I think this should be no problem.
Get Logic X and Mavericks and the fun begins.
Logic X is very effective on resources, and even better now with Macericks.
Its fine. Just make sure you have some decent ram in there. 8gb will do it,
thanks guys! your help is really appreciated.... So my daughter will get an iMac and I have less trouble with Windows 
Cheers
Sacapuntas
If you haven't already bought it, there's also the option to choose fusion drive, a mixture of a conventional HD paired with 128 GB flash.
I replace my stock 5200 drive with a SSD from crucial and a nice 2TB 64mb cache 7200 harddrive.
I have an i7 imac and now the machine really fly's. The change is not that difficult if you follow this guide.
http://www.ifixit.com/Guide/In…+Dual+Hard+Drive+Kit/7575
I always felt that the drive was holding my system down, a very fast cpu, lots of ram and then a verry sluggish harddrive.
Yeah, SSD are amazing... the system boot ups in seconds. Unbelievable!
With the third generation they have also addressed some of their youth issues.
Still too expensive for replacing big-sized HDs, but for boot, cache, samples' libraries and realtime recording they're great 
