ok. i know that. but my doubt comes because in asrock site there's a list of supported gpu for crossfire about my motherboard and it sound weird that there's not 5xxx or 6xxx series...i google it and i found only questions like mine..by the way my card support crossfire and sli but i'm not sure if supports 5xxx or 6xxx series.
as you can see http://www.asrock.com/mb/vga.it.asp?Model=P55%20Extreme&c=CF
they don't mention 5xxx or 6xxx series. i'm thinking about upgrading or with another 4890 or with 2 6850 but i want to be sure before buying...
anyway thank you very much.