You can point your GoDaddy-registered domains to your Dreamhost account by editing the DNS Servers over at GoDaddy to point to Dreamhost:
Pointing the domains at DH means the management of domain records will be done at GoDaddy while the files are hosted here. Transferring means Dreamhost will be your registrar and you will be able to manage your domain records right here in the Panel rather than having the hosting here and the management somewhere else. If you're getting a somewhat sizeable discount on alot of domains then it might pay to have your eggs in two baskets. If not, it's probably easier to have your stuff in one place for easy access.
When you transfer your domains over here they actually give you a year's domain registration on top of it (value $9.95 each). Most registrars do that these days. Something to think about if costs are your primary concern.
Maximum Cash Discount on any plan with MAXCASH