Outlook keeps the information about which emails have already been downloaded in the .PST file (for versions after 2000). However the information is linked to the specific email account via a unique ID. When you recreate an email account (in the same or in a new profile) then it gets assigned a different unique ID from the original email account
Hence, although the .PST file contains the information it cannot be linked to the email account as the IDs don't match.
I have found a piece of software called MoBackup ( www.mobackup.com ) which allows you to backup your email account info and "restore" it to a new profile or new PC with the same ID as the original account. (using their "Restore Outlook Profile 1:1" option)
You only need to do this once and you can then copy the .PST file to the new PC / Profile and all the information about which emails have already been downloaded is preserved.
Note that MoBackup won't restore the passwords assoicated with your email accounts (citing security reasons) so you need to re-input the passwords after the restore.
If you want to use MoBackup to copy email account information from one PC to another (and keep the same unique ID), then the version of Outlook on both PCs has to be the same.
I have tested this on Outlook 2007 and it works brilliantly.
The software is not free but worth the money I reckon.