Unlike some OS which have special 'hybrid' iso images that also include an HD-like partition table, OpenBSD's are simple CD ISO images.
For 5.5 (not yet released) there's a dd'able install55.fs image. If you want something to try now there are -current snapshots which also have this; they are *newer* than 5.5 so you will either need to reinstall to 5.5 later if you want the release (downgrades aren't supported), or stick with -current until 5.6 (which will require occasional updates of base os + packages). Testing -current to see if your NIC is detected would certainly be a good idea even if you prefer to run releases.
