I would believe that SUN (or any company making and selling hardware AND software) would insure that you could only use their parts with their software. That way they insure that their company can survive and grow.
Solaris X86 would support your hardware as far as the card but would not work with Sparc processors. Solaris for Sparc would work with everything but the card (more than likely).
About the only thing you could do (if no one has a definitive answer) is backup the system, put Solaris on and see what happens. Or replace your card with one made/supported by Solaris for Sparc.
Components for Ultra 5