How does Mail 4 organize folders for IMAP vs POP in sidebar?

Can someone explain how Mail organizes the sidebar for POP vs IMAP and what would happen if I switched totatlly to IMAP?

I'm a bit confused by how Mail is organizing the folders in the sidebar for an IMAP account. Way back in Panther days I had an IMAP mac.com email for a year or two and if I recall correctly I saved it in On My Mac as a folder. Same for a POP account I was getting rid of.

Since then I have had only POP accounts (including a gmail account). I managed to keep the inboxes (at least) in sync by not removing mail from the server so that my ipod, iphone, and other Macs can log into the mail server too. This has worked OK. However, since I send a lot of mail from my iPhone, I'm wondering if it's time to switch to IMAP since I will be giving up several ISP issued email addresses that only use POP.

I signed up for a new gmail account recently and it set up as an IMAP account by default. Now there's a folder for the new IMAP Gmail account at the bottom of the sidebar under On My Mac folders. It doesn't have an inbox or sent folder but does have a "sent items" folder.

Additionally, at the top of the sidebar is "Mailboxes" with Inbox, Sent, Drafts, Trash and under these are a subfolder for each POP account and also one for the new IMAP gmail. It seems weird to have the inbox and sent items so far away from the rest of the folders. I'm also not clear if it's keeping 2 copies on my hard disk one in the top mailboxes under "Sent" and one at the bottom Gmail folder under "sent items".

Finally, I'm wondering would it be prudent to back up all of the existing POP mailboxes or move everything to On My Mac before switching some of the other mail accounts over to IMAP? I will want to keep archives of the ISP emails when I cancel my service with them.

Thanks for any clarification and advice.