Right you have a 4X board mate, didn't realise your system was that old.
90% of 8X cards work on 4X boards, I think your board will pump 1,5v through the AGP slot where as 8x cards use .8v, not a problem though as the card will handle 1.5v without any problems.
Some people in the past have had issues where their 8x card does not work on their 4x board, but these are few and far between.
If I were you I would purchase the card and try it, keep the box and everything neat and tidy. If you have a problem then just box it up and RMA it while ordering a new board. If it works then you have got a result via the cheapest option.
If it does not work then return it and ask them to add the cost to your new order, then order a new motherboard with PCI-express slot. (That is if they do them in socket 478), and also get a PCI-express card.
But if it were me buying I would try this card first and take the gamble, after all your in a no lose situation as you can return it at their cost.