about pm box storage, I have this suggestion (it depends on whether vbulletin)
forward pm to email
and auto-fwd reply from email to pm receipent (email address will be filtered)
that how it works:
pm sent from a to b
pm fwd to b's email directly

when b reply the pm->sent
pm forwarding gateway receives the email, read the headers, and forwards the pm to a

that seems like a good idea even though i haven't got any PMs :/

can you explain this a little better. Im not gettin what you mean!

like a PM to email and being able to respond via email

this is what I talk about
like I make a pm to arigold, the pm will be forwarded to arigold's registered email
after he replied and sent, the email will be going through a gateway, detect the header to determine the receipent, and will be sent to either pm inbox or forward again to my email (depends on my preference)
if vbulletin has this function, this is pretty handy
-----or-----
the way aximsite does:
if a member to send me a pm, I get an alert through email, and the email displays the content of the pm, that I am able to decide to reply or not