I know that similar question was already asked but there are no information about how serious is this message. I mean, does it mean that my ATI 6750 card wasn't really recognized by proprietary driver or it is a bug? In the ubuntu documentation it is written that this card works from box. So I'm a bit confused. PS I installed "AMD/ATI FGLRX proprietary graphic driver" but I can't install "AMD/ATI FGLRX proprietary graphic driver(post release updates)". It returns error
Unsupported Hardware Watermark
This can happen if your card's PCI ID wasn't officially certified to work with a particular version of Catalyst. It does not necessarily mean that your card is unsupported, but it does mean that you shouldn't file bugs with that particular card/driver combination. If you installed the driver by downloading it from AMD/ATI, installing a newer version of Catalyst will probably help.
If you installed the proprietary driver included with Ubuntu or you do not want to upgrade to a newer version, it may be possible to work around the issue by using a control file from a newer version of Catalyst than the one you're running.