sunny - If you have control of your old server, your site needn't be down at all. Simply having operational copies of your site on both the old host and the new one, and then change your A-records in your DNS.
As the DNS propagates, either the site on the old IP or the new IP should resolve. I tend to keep the old site up for a while after the propagation is complete... but which version is seen depends on which DNS is seen. The trick is to have both old and new sites operational at the same time.
If you don't have control of the old server, though, or if you're also changing domain name servers, then the propagation won't be seamless, but in my experience the propagation is now much faster than two days.
Similarly, if the site is highly dynamic and interactive, it might be easiest to suspend operations for a short while.
The correct header response for a site under maintenance is a 503, and we're currently discussing it in this thread (along with a bunch of other issues that fortunately won't concern you)....
Hacked website - recovering from Google SEO perspective? http://www.webmasterworld.com/google/4514373.htm [webmasterworld.com]
PS: I wouldn't make site changes at the time of the move. If you are planning to make changes, I'd move the site first, let it get established, and then make changes.
