That's what I'm thinking. The last time this happened, it was a USB 2.0 PCI card that made my ASUS P2B refuse to POST in the exact same way, but that PCI card works perfect fine in THIS Abit KX7-333 motherboard. I think maybe the card was made for an 8x only AGP slot, even though it has the AGP 4x 1.5v notch in the card. And I don't actually have any other MB that has an AGP slot to test the card in other than an AGP 2x slot on the ASUS P2B...so I have no way to test the card myself. The fan on it spins. I know that much.
Perhaps it's because the GPU chip is designed for the PCI-Express bus (It even says PCI-Express on the chip itself when I cleaned off the thermal paste during the breakdown and clean). It probably has a PCI-E to AGP converter chip somewhere on the board to allow it to use the AGP bus...but wouldn't that also limit it to AGP 8x operations?
It may be that I'll need to flip the card on E-bay, reclaim most of my money, and try an ACTUAL AGP 8x/4x native card, such as the original 9800xt I originally wanted (but people price-gouge on). Hell, in THEORY, maybe I could make a slight profit off if THIS card in order to ease the purchase of the other one?
It's very possible it is to do with the bridge chip, but it won't really help solving this problem. Best bet is to indeed get another card (perhaps be patient and spend time on another project till something good comes up?).
If you really want the card itself (the X700 you purchased), do you think it's worth the trouble to sell it just so you end up with nothing?
But in the end it's your decision.
Btw, you don't have a single AGP 8x board? I don't want to tell you to do things in a way that's not your way, but one of the benefits of having a more extensive collection of parts, is that when it comes to building rigs, some things are simply not made to match.
I've had strange compatibility issues many many times, stuff that won't work (even though it should) or keeps giving problems that seem impossible to solve. At some point it just seems easier to simply swap it for something else and be done with it.
I'm not sure how good the Radeon X700 series is (never even seen one in the flesh I think?), but s754 and s939 are great AGP platforms.
Personally I would keep the card (unless you can actually make a profit, as you already mentioned). I would not sell it at a loss though as it would leave me with time and effort spend and nothing else in return.