The Labs web pages indicate a basic hardware compatibility.
If you check the archives, then more information is available.
In the most recent update, Voodoo was added. It was developed
on a 3000, but should work with others.

Plan 9 so far has not made use of 3D, so good 2D performance
is all that matters. This, plus the relative difficulty of getting
specifications for the market leading 3D cards makes keeping
up with current hardware tricky.

Currently the big holes are

ATi >= 128 bits: documentation is easy to get, but no-one
	with the skills/hardware/motivation has stood up to
	be counted yet

Nvidia: known to be hard to get documentation

Matrox G series: reasonable documentation can be got.
	I have got a G400 working at 8 bits depth, but time
	is short

As for on-board, if you mean SIS, then no.

Sparcstation 5. Not immediately; the latest release is only tested
on x86. I have one lying in the corner of the room as well.

I would download the 3rd edition manuals from the website and
read them. There are a _lot_ of changes. Put it this way, I don't
read the old manuals, pretty though they are.

Nigel