I can't be sure but that board does seem rather old and probably wouldn't support an 8x AGP interface.
This doesn't mean it wouldn't be able to use a card as powerful as that since it'll still support a 2x AGP interface which the card would lower down to.
This wouldn't cause that many problems either though, except the card might not run as fast as a motherboard running at 8x, and some effects might not work.
For example Need For Speed Underground when I had a 4x interface on a 8x card, the motion blur didn't work, but as soon as I updated, this kicked in.
I wouldn't get a card as good as a x850 in that case, I would go with a Geforce 6600 GT in a PC like that. It will run latest games rather well too. Or a 6800 like 99tollop says