You might find one of the Notational Velocity forks to be exactly what you're looking for:

http://notational.net

http://brettterpstra.com/projects/nvalt/

(nvALT adds Markdown support and a few interface adjustments)

Point whichever NV you use to a directory of txt or markdown files in your Dropbox. NV is nimble and lightning quick, and it excels at super-fast search. Don't even bother with tags, just search.