The 7800GTX is PCI-Express. That means that really, you are only looking for motherboards with the Nforce4 Ultra chipsets or the Nforce4 SLI ones. You could also get a VIA K8T890 chipset based motherboard although the performance is better on the Nforce4. I would recommend the Nforce4 Ultra chipset unless you plan to use two cards in the SLI mode. If you wish to have that option open, go for an Nforce4 SLI based mobo. Ok now....you could have a look at www.newegg.com and look for motherboards there. I recommend the DFI LanParty UT NF4SLI-D.
The dual (SLI) setups usually bring performance increase of what I guess to be around 30%. In some operations, the performance is much greater. However, in most cases the extra paid for the second card is not justified by the performance increase. You will never get twice the performance in games etc. with two cards