Are you considering any particular cards ?
I have had good luck with DEC21040 (10baseT) and DEC21140 (100baseT) cards
in Solaris 7 through Solaris 10 on x86.
The same can be said for 3Com 3C905C-TX cards.
Solaris 8x86 will recognize many chipsets natively,
and a number of third party developers have created working driver packages.
Most of their driver packages are free, but a few are not free.
Solaris 9 works with a few more, and Solaris 10 works with yet more adapters.
The <a href="http://www.sun.com/bigadmin/hcl/sol8/" target="_blank">Solaris 8x86 HCL</a> list has the native cards.
This <a href="http://www.sun.com/bigadmin/hcl/indexRes.html" target="_blank">other Sun web page</a> can direct you toward 3rd party sites.