Claris Emailer is great for older machines since it is small and fast. This is a great solution if most of your email is text based. I don't think it will display html, but I could be wrong. It handles multiple email accounts well. The Lite version was free but the full version was not. I'm not sure if the lite version allowed multiple accounts.
Eudora is top notch, and would be a better choice if you need html display. I'm not sure if they still offer a free version. You might be able to get an older version like 4 that someone might give away. The older versions are still very capable and may be better suited to an older machine and OS.
If your looking for the "best bang for the buck" free that is I would have to say Eudora its about as full feature as you can get at N/C. Although I'm not a huge fan of M$ OE may do the trick. If you don't mind spending a bit of money take a look at Mailsmith by Barebones Software www.barebones.com I tried it out before have to move to M$ Entourage as I'm in an Exchange environment but I was really impressed with the demo.