The most important thing to take away from this answer, is not
the fact that it doesn't run at 8X. 8X is really no faster than
4X - see some of the articles on the popular review sites for
some actual benchmarks.
The important thing is interoperability. If you connect an 8X card
to an AGP 2.0 spec Northbridge, it reads it as a 2X max card, due
to the difference between the way the status register is coded in
AGP 2.0 spec and AGP 3.0 spec. So, the mismatch will be more
annoying that you would expect (buy 8X video card, get 2X perf
if you buy a AGP 2.0 spec Northbridge).
If you combine a 4X max video card with a 4X max Northbridge,
everything works as expected.
If you decide to look for another 8X Northbridge, you should
still download the manual and read the BIOS setup section.
Check the AGP setting part of the manual, because some
Northbridge chips only do 8X and 4X and cannot be set to
2X. Imagine what happens if you buy a 4X video card and want
to slow it down to AGP 2X, to gain stability - you can't.
Check the manual to make sure the AGP settings you will need
are actually there in the BIOS.
The best combo, is to match "like to like", as you get the
most matching setting that way.