How do I archive old emails?

First, a question: how much of a performance decrease am I going to see as old emails build up? At what point does that happen (i.e. when I have X megs of emails stored in my TB folders)? Second, is there an easy/safe way to archive my old emails? I have old emails relating to eBay and other online purchases, newsletters, etc. that I don't access frequently, but would like to be able to easily go through them at a later date. I have plenty of space on my flash drive (2 gigs) and would prefer to keep them there so they're always available. Backing up to my home computer is a decent second option. Mostly I'm just worried about slowing down TB because I'm hoarding old stuff. Any suggestions?