I always thought that huge .pst files are drastically slowing computers down because they take most of RAM. Since .pst is only one file then it has to be whole uploaded to RAM for Outlook to be able to use it. Isn't this correct?
No. The whole pst isn't loaded into ram. A huge pst can slow outlook down, but its more when you change folders, because outlook has to redrawn the screen and load the messages. I usually recommend keeping the inbox under 5000 messages, because that is the most used folder.
Which configuration is better in terms of internet bandwidth; Downloading full messages or Headers only? There are around 25 computers on the network, average download speed is between 7 and 9 Mbps.
It really depends. If you don't need most of the bodies, headers only is better, but the bandwidth used is about the same otherwise, with headers only spread out more, as you do a burst to get headers then load bodies as you need them.
Does it mean that when I set up IMAP account to download headers only then when I open email is it opened on the server or is it actually being downloaded to the computer's hard drive and next time I open it again it will be opened from local hard drive?
If change settings to "Download full messages" will I be able to use that IMAP .pst file for a backup of all emails that are on a server (by copying it to external drive)?
When you use headers only, outlook only gets the header until you open the message to read it, then it goes out to the server and grabs the body. The body will stay in outlook for at least few weeks - if you don't view it for a few weeks and outlook refreshes the pst, it'll remove the body. If you get full messages, yes, you can use it as a backup and open it in any outlook profile to read the messages.
This most problematic Gmail account has 32GB (there is a lot of graphic attachments in most of emails). It's around 3 years old. For this sort of account is it still better to configure account to "download full messages", "headers only" or is there a different solution?
Are the messages all in the inbox? Make sure you don't subscribe to the All Mail folder - that will download everything twice. If you don't need to view the old messages or don't need a backup copy, I'd consider using headers only.