i'd recommend going for something with PCI-E since you need a new mobo anyway. Don't go for AGP. you'll get screwed with a 7800gs :roll:
i believe asus has 3 different pci-e mobos,
here is one i got:http://forumz.tomshardware.com/hardware/ASUS-P4GPL-478-PCIE-Review-ftopict172519.html
However, if you need more than 2 dimms of ram you might want to choose a different model.