If you already have a DreamHost plan, simply add the domain to the DreamHost system as 'fully hosted' using the admin panel (Domains -> Manage Domains -> Add New Domain / Sub-Domain), then use whatever configuration utility that Yahoo provides to change the name servers for your domain to...
This will leave the registration at Yahoo, but the site will be hosted at DreamHost. Later, if you wish, you can transfer the domain registration to DreamHost as well.
Note, it can take a day or two for the DNS information to fully propagate across the Internet.
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