Just curious.  I have X2 but I've recently put a 128Gb SSD in my Laptop and want to keep it lean.  I typically record out the eSATA so audio files aren't the issue.  I had 200Gbs of programs on the other HD and just want to be careful.  I do have Reaper and MixBus installed.
 
My question is which version of Sonar would be best to load?  I'm not sure of the footprint of X2 but was reading the incompatabilities of VS with newer Sonar files?  X2, Studio, Essentials, Creator? 
 
I will mainly track (not mix etc. on the lappy).

Since your concern is about footprint then it depends on which level of X2 you plan to use. When I downloaded the trial version I thought I was downloading the version I was looking at - 'Essentials', but put Producer on my tablet anyway for the trial. It worked fine for the most part (enough to convince me to buy it). I then picked up the V-Studio with its software and put that on it too. It is a much smaller program, but I'm not so impressed with it. I believe X2 Producer takes somewhere up to 40gb for a full install, not too lean for 128gb considering your OS takes close to 20gb.
As I said, it depends on your needs. You don't have to do a full install or split it up for the DAW on your main drive and the VST/softsynths/samples on your other drive or esata.
You could just go with X2 studio or essential and only install what fits your needs of any of the versions.

I would mostly compare VS to garage band, but for grown-ups. X2 is a real DAW, VS is a toy (but it's quite good for a freebie). Choose the software that best fits your needs then setup your computer to accommodate, going the other way around is much more headache than many people realize.

I use Sonar X2 myself.  My laptop had XP Pro on it and it ran fine under that.
My desktop has X2 on a Win7 64 with 8 GBs. of ram and it runs great on that.
I've recorded up to 4 analog tracks at once while having a drummer softsynth running on 16 or more MIDI tracks.
No problems