Bulk downloading of map tiles from openstreetmap.org is strongly discouraged. See http://wiki.openstreetmap.org/wiki/Tile_usage_policy
If you want maps at a high zoom level for a large area, you should render them yourself, or use another service.

Plus rendering them yourself means you can customise them, to only show the things you want.
Using bitmap tiles then tracing them is a much more complicated way of doing this!

Or you could look at Cloudmade Maps http://maps.cloudmade.com
They have an online style editor, so you could customise that, then use their tiles. Though check their terms, as to what they allow for downloading large areas.