Hi Sheila,
Glad to have you as a user!
APC is what is known as an opcode cache. That means that instead of PHP having to compile the code into machine instructions every time someone visits your site, APC stores that compiled code to save PHP having to do the compilation every time. So it's very different to what Falcon does which is a "page cache".
Falcon stores the "rendered" version of your page which is what gets sent to the visitor so that your site does not have to generate it every time. Falcon is designed so that when a cached page is served, your site doesn't execute any PHP at all - the web server just serves the cached page directly from disk without even executing PHP. So Falcon can make your site even faster than APC can because we save you from executing any PHP at all - rather than making PHP execute faster which is what APC does.
Hope that makes sense.
Now, regarding optimal settings. This really depends on how your site is used. I would start with very generous settings - particularly in the login security area (20 failures before lockout) and the Firewall Rules area (set everything to Unlimited to start unless you're solving a specific problem).
Let me know if I can help further. We're always happy to gain another user, so welcome to the family.
Regards,
Mark.