'Guaranteed' data rate down Gigabit Ethernet?

I know this is a dumb question, but I'd be grateful for any numbers at
all; I'm not sure where to start at the moment.

I need to get video data out from a PC on to gigabit Ethernet, on a
point-to-point link. The video will probably be pulled directly from a
graphics card, and will end up on some custom hardware at the far end
of the link.

If I wrote a driver to do this, sending raw data and ignoring the
TCP/IP stack, does anyone have any idea of what data rate I could
sustain from an "average" PC, whatever that is? Would it be possible
to guarantee a minimum data rate, or could there be arbitrary
interruptions to the data flow?

Second, do you have any idea of what these numbers are if I just used
an off-the-shelf driver and TCP/IP? Could an off-the-shelf driver
actually saturate Gigabit Ethernet anyway, over an extended period of
time?

Thanks -

Tom