I don't think WordPress hosting requires Windows or their is any difference between Windows based Wordpress and linux based WordPress. So you can move your WP from Windows to Linux.
I don't know about other sites but it all those needs Apatche, mySQL than You can host those sites otherwise you need extra effort and money to switch from Windows based server to Linux based server.
Dreamhost is much better in terms of downtime. But remember If you need more RAM or CPU consider switching to DH VPS, Not on shared hosting. You may try with 97 days money-back guarantee. I have made some review about DH shared hosting, You may visit my blog NetRival.com (and search "Dreamhost") to read my posts.