i think there are some 478 915 boards but they only work with prescotts, so your out of luck. I beleive asus made one, but didn't sell it in the us. I had the same idea as you a while back.
How long can I go before I upgrade my motherboard?
Your Northwood CPU will likely keep you in the game pretty well, but PCI Express support is mostly unnecessary at this point. Wait until your next system-wide upgrade if you can.