I set up a client with a new laptop (Windows 7, Office 2013). I took 3 old PST files from his old machine (Windows XP, Office 2003) and consolidated them into a new single PST file on the Office 2013 machine by selecting individual folders in the old PST files, and using Move To...
to move them to the new PST file. The old PST files had a combined size of 4.5 GB. The new PST file is now 47 GB in size
I used the "Compact Now" process that Microsoft has posted here: http://support.microsoft.com/kb/289987
. This had almost no affect on the file. (It's still 47 GB in size.)
It's worth noting that I backed up the old PST files before I gutted them. Deleting the 47 GB PST file and reconnecting to the old files is an option.
I personally do not combine all items down into 1 single PST. With heavy mail users, I usually have them split it up into quarters of the year and we archive any of that off to DVD for restore purposes only. If you follow this format, PSTs of the past will never change, thus you can back them up to physical media once and always have a valid copy (no new items should ever be placed in the PST). While they may have multiple PSTs to now deal with, they do not have to search through years of mail just to find items from a particular date range and they do not lose all of their items should something happen to a file.