> My questions are these:
> 
> Since I will not be connecting via NIC, I do not need to configure it and

well, unless you set up your linux box as a firewall this is true.
But, given your other email about doing firewalling/nat/proxy,
I'd bet you are goin to set up your system as a firewall/proxy/whatever
and want that ethernet card to work ;-)

but you are right, ehternet card does not affect connecting to your
ISP.  (directly)

> all I need to do is:
> 
I'll let others answer those.
> 
> -->> Now what happens to my mail if I'm only connected 5 or 6 hours a day?
> Will it bounce or find it's way while I'm dialed in?


Depends upon the other end.  Some (MISCONFIGURED) email transport
agents will bounce email that does not make the next hop (usually
to the final destination) within 4 hours (!!!).  This is, in
my mind, completely bogus, but hey, that's for another flame..,er
thread ;-)

What WILL happen is the external MTA will attempt to deliver the
mail.  If you are not dialed in, it will put it in the 'deferred'
queue (or equivalent), and try again at random times (usually
for 4 hours before notifying the original sender that its having
problems delivering, and for 5 days at which point it will bounce
it back to sender).

Notice the key phrase "try again at random times"!  Some MTAs use
1/2 hour, some use 1 hour, some truly seem to use random times.
When I was first considering this I figured I had to stay online
for at LEAST one hour per day, and really I needed to stay on
for 2-4 hours, and even then I could not be sure I'd keep all my
email from bouncing.

So, What to do, what to do?  Go ahead and set your email up
as you propose, but don't tell too many people about it until
your MX machine is on full-time (or nearly so).  Test it and
all, but use your current email address, and either forward
all mail to your home machine (using .forward - see man
pages for sendmail I think) OR continue reading mail the 
way you do now (mail client on your ISP or POP or whatever 
from home).

Hope that was clear, its early and I'm not fully awake yet ;-)

rusty