I think you are looking at £159 to upgrade from Sonar 7 producer to x2 if you go to Cakewalk directly
You can also get it for £135 from here in the uk:
http://www.etcetera.co.uk/?homeThe new version takes some getting used to but it is awesome - the upgrade price is worth it for the Pro-Channel on its own, let alone all of the other new bells and whistles
I can also help you get up and running with X2 as I use it for all of my stuff (and I wrote the review for Future Music magazine
)
I think Sonar 7 will work but it is a few versions behind now (8 - 8.5 - X1 - X2) so there are lots of new features that might make your life easier
The main concern is making sure good drivers are available for all.ofnyour hardware, especially your audio interface!
PM me if you want any specific details