I'll be a nice b3tan
and actually answer your questions:

The change should be done within 20 minutes usually with an automated system, but you will almost certainly be transferred over to a new server, this in turn will cause your DNS records to change to the new server IP.

The FastHosts DNS should be updated within minutes (but can be up to 4 hours on standard DNS update timings), but the rest of the world needs to catch up, so not everyone that connects to your site will see the change instantly, and can be up to 24 hours to fully propagate.

Your mail should be unaffected as these are usually (and should be) on different servers.
(Ethelred is watching cheese grow, Tue 23 Mar 2010, 10:45, 2 replies, latest was 16 years ago)

Aw!
Aren't you lovely.
( girlinthehole, Tue 23 Mar 2010, 10:47, Reply)

lovely a fucking loser
( TGB checking Off Topic is still shit at, Tue 23 Mar 2010, 10:47, Reply)

You're just jealous
As I have bigger tits. Namely Al and Bert.
(Ethelred is watching cheese grow, Tue 23 Mar 2010, 11:11, Reply)

This is instructions for turning your computer into a horse porn server

( Naked Ape call me Caitlyn, Tue 23 Mar 2010, 10:49, Reply)

Well, the website in question is www.horsegaggers.com

( applehead herp derp, Tue 23 Mar 2010, 10:51, Reply)

Thankyou very much :D

( applehead herp derp, Tue 23 Mar 2010, 10:50, Reply)