Saving Outlook messages

Had to have a technician reformat my hard drive, so am starting all over. I'm trying to get things a bit more organized this time, but am wondering about the best way to save large numbers of email messages from Outlook 2000. Since I sell on eBay, I have large numbers of emails which need to be saved each month. I'm not so good at cleaning things out regularly, so over a period of time, this can amount to thousands of messages.

I seem to recall reading somewhere that it was best not to save large numbers of emails as folders within the program, but to set up folders direct on the C drive instead. Is that correct?

And in what form should I save them? I see I have a choice of HTML or Message Format? (Choices also include Text Only and Outlook Template, but those wouldn't be logical choices for many.) What are the implications of choosing one or the other of these?

Any other suggestions for how to get things organized?

I have plenty of unused hard drive space, so that is not an issue.

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Dell Deminsion XPS T600r, Pentium 3, 26G HD (4 used), Windows 98se, Outlook 2000