The most likely cause for Dreamweaver not remembering your FTP username and/or password between sessions is that Internet Explorer 7 is installed on the same machine.
This is a known bug with Dreamweaver 8.0 & has been resolved in the 8.0.2 Update that's downloadable from Adobe's Web site.
To download the update, go to:
http://www.adobe.com/support/dreamweaver/downloads_updaters.html
WARNING: You will need the original installation CD to complete the 8.0.2 update. Although the update's documentation talks about a workaround, it involves downloading the trial from the Web, which is no longer available due to a later version now being offered.
For more information, visit Adobe's KB article on this topic:
http://kb.adobe.com/selfservice/viewContent.do?externalId=3491671c&sliceId=2
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