Thanks for the input guys, unfortunately all the techie talk is waaay above me...!!
It's a mid 2011 iMac, that's all I know. They all look the same to me..!
I did ask for a 2TB drive when I ordered it so that's the only non-standard addition. I regularly update it and it works fine, much faster than either of my Windows laptops.
I'm in the UK at the moment and the Mac is in France so I can't help with further info.
Getting back to my original question, how important is the inclusion of RB in the version I eventually buy...?
As you know, I don't want to spend unnecessary hours on the technical side of things, I just want to create my backing tracks. I'm far more of a musician than a computer guy. But I'm forever grateful for guys like you who understand these dark arts...!
If RB makes my life easier, then that's the version I want. I'll sort out the hardware to actually do it later.