For the moment, what you can do is create a workspace for that client or contact, and store there everything related to it. The problem you have with this is that you will need to duplicate some info (the contact's name, in the contact and in the workspace).
Otherwise you can use a tag with the client's name (worse solution than the previous one).
The last thing I can think of is, when you are writing the note, expand the Linked Objects category, and link that note with the contact or company you want. After you save the note it will be linked with the contact. You can then access it by viewing the contact and looking on the linked objects list (properties panel, bottom right). The problem with this solution is that it will not be easy to navigate the info related to that contact.
The reason why i said "for the moment" is because we are already designing a new concept that better fits this kind of situations, that we will introduce to OpenGoo some versions from now.