SharePoint has native functionality to synchronize Active Directory information via User Profile Synchronization (which is essentially a lightweight version of Forefront Identity Manager that's bundled inside of SharePoint). That would likely be the method you'd want to use to do this, not via PowerShell. Pushing updates via PowerShell may be erased when the next UPS sync runs, so you're far better off having SharePoint manage all of the data.
If you're unfamiliar with UPS...
User Profile Synchronization (UPS) is a service application that gets provisioned within SharePoint Central Administration, pointed to the applicable domains/OU's, and synchronizes on a schedule (ie: every night) to bring AD information (manager, title, etc.) into SharePoint. There's then an hourly timer job that pushes that UPS data to the individual site collection user information lists (you "shouldn't" have a need to touch the individual SPUser objects to do this if your farm is setup correctly. On a side note, you can also setup custom fields and mappings, or use SharePoint BDC to also synchronize to third party HR systems for fields, as needed.
If UPS isn't setup, Spencer Harbar has one of the best guides to implementing UPS available on his blog. UPS can be a little picky, and has to be setup exactly in the right order or it won't work, so follow his guide and you'll be good. UPS has a tenancy to need to be fixed or reconfigured after Service Pack installs simply because it is so finicky, but once you've done it a few times it's simple. If you're running Service Pack 1, you'll want to make sure you're also running at least the June 2011 Cumulative Update as it provides some additional fixes to UPS that were missed in Service Pack 1.