To find the commit hash for a particular svn revision you can do
git svn find-rev r800
However, that will only help you once you have the subversion commits in your git repository. The best option is generally to get a checkout that includes the entire subversion history.
git svn clone http://example.com/svn/trunk
Or if you want, all branches as well as trunk
git svn clone -s http://example.com/svn/
If you want to continue down the current path (which will be painful) you can get a second git repository that includes more revisions than the current one, and then graft them together using git grafts. So for example if you are certain that you only want revision 800 you can do another
git svn init http://example.com/svn/trunk
git svn fetch -r 800
However, if you then want revision 600, 400, 900, etc, you will forever have to mess with your local git repository. Better to just start over, get a proper git svn checkout that includes all revisions and then you can freely check out arbitrary revisions without having to jump through hoops.