How far the US goes, really depends on the will of Americans. If the will breaks, Afghanistan could become a vietnam. On the other hand if the will doesn’t break, Afghanistan will be much more stable and civilized.
All in all, a stable and civilized (or less tribal) Afghanistan will be much much desirable from Pakistan POV.
Operation in Wazirastan so far has shown that Pakistani tribes are ready to settle down and become urbanized. That’s why Talbhianic pigs have not received large scale support from the general population.
Perhaps Afghani tribes are the same too. If Americans army learns from Pakistan, they too will succeed in the Eastern and South East of afghanistan. This will perhaps for the first time in the bloody history of Afghanistan, make that gad awful region a single cohesive country.
And that seems to be American goals to make it a single governable country and have a pro-West leader running that country.
As described above, it really depends on the will of American people, and craftiness of Obama admn to achieve these goals.