Dreamhost generates DNS entries for your hosting on dreamhosts nameservers. Since you have set other name servers these will never be referenced outside of dreamhost.
On the 'manage domains' page of the panel, the listing for your domain needs to say "Fully hosted". Once that's true click the DNS button underneath the domain name. You will see the entries that dreamhost is generating on it's nameservers. The first should be an A record with an IP address, that's the IP address you need. Add an A-record pointing to that IP at your registrar.
NOTE: Shared servers have multiple IP's. If you have multiple domains/sub-domains on the same server they may have different IP addresses.
it could, but they don't very often. Unless it's done on an emergency basis, you should get an email about your server being changed before it happens. Even when that happens it's pretty seem-less because there is overlap, that is both the new and old IP will work for awhile.
Just noticed what I bolded. Don't follow that list, he's doing something else... his registrar isdreamhost and he's changing nameserver elsewhere, then pointing back to hosting. Your nameservers are already pointed where you want them at your registrar.