As far as I'm aware, the plan is that Class 334s will be cascaded off the Ayr and Inverclyde routes and onto the North Clyde to displace the Class 320s and to operate services along the Airdrie-Bathgate route. Class 318s will also supplement the 334s, and I think the plan is to keep 318s on the Argyle Line with 334s supplementing, so basically it looks as though they will be interworking with each other on similar diagrams.
Reason why Class 320s are going to be displaced is down to the new line being 80mph, which the 320s cannot do due to being 75mph units, but the 318s and 334s can due to their 90 and 100mph top speeds respectively.
Only problem is that a 3-car 334 has less seats than a 320 due to the size of the loo's in the PTSO and the 2+2 seating arrangement in the DMSOs, and the fact that 320s do not have loo (again, another reason why they're going to be cascaded)...
318 = 216
320 = 227
334 = 183
The original plan would've meant that the 320s would allow the Class 314s to be withdrawn. However, I have since been told that the 314s will now not be withdrawn and will be around until 2015 - work is being carried out on them to make this possible.
Whether a Class 380 will operate north of the Clyde I do not know. ScotRail's publicity suggests to me they won't, although I have noticed that the ScotRail EMU fleet has a tendency to end up everywhere, so I guess it will be a matter of time.