Is there any video card, which would be as fast as nVidia GeForce 8800GT or faster, but would have open-source fully-functional (i.e. including 3D acceleration) drivers for linux, and would cost not much higher than gf8800gt?

ATI may someday meet those requirements, since they released the Specs for their drivers, but at this time I don't think there is any 3D graphics card that has Fully open source drivers and offers the level of performance you would get from a high end nVidia or ATi card.

Answer: NO

But, maybe someday, nouveau will be usable (lets say several years from now).

I'm not quite answering your question, but here is a perspective:

If my machine works, and I am free to choose how I support the community (ie I don't have to pay for anything directly), then why do I care if my video driver is open-source?

Example: Nvidia seems to be doing a very good job supporting Linux---I have 2 machines with Nvidia cards, and it does not bother me one bit if the driver is closed-source. Nor would it bother me if the only driver for my printer or scanner was also closed. Ditto for the firmware for my wireless.

The point of ANY OS is to have a system that works--period. The typical user may appreciate the benefits of OpenSource, Choice, etc., but will typically not respond well to the OpenSource purists.

<<end sermonette--stow soapbox--get back to work>>

So there you have it, Buy that nVidia card, install the proprietary Drivers, Get great 3D and OpenGL performance on your Linux box, and be happy like the rest of us

Well, I do care, and that's for a reason.
Hmm... I didn't ask whether i should install proprietary drivers or not... I just need an ability to change the code of the drivers, optimize them using different compilers and their switches, etc., while, of course, not having to decompile/disassemble and recompile them .
I already have proprietary drivers installed and now i'm just searching for a replacement which would give me the needed options while providing with good enough performance.

Join the nouveau project, that's the only option so far.

Should have specified the reason in the original post. It would have made the replies more in line with your intentions..

Without the reason for the request all we can do is speculate..

Cheers !!