I'm going to be the odd ball person here. Have you considered something like this?
XFX PVT70KUAD7 Geforce 7800GS 256MB 256-bit GDDR3 AGP 4X/8X Video Card - Retail
or this for a little cheaper version (rebate)
eVGA 256-A8-N506-AX Geforce 7800GS CO 256MB 256-bit GDDR3 AGP 4X/8X Video Card - Retail
I know you're looking into ATi cards, but this is just an alternative and I don't see any reason why your motherboard can't support it. It looks like a damn good card to me, especially running on the older AGP model.