I'll share my experiences from hunting round for a decent Core 2 Motherboard for which I have spent 2 months doing and in that I have acquired a fair amount of knowledge so I hope this helps.
SP you want a Core 2 supported motherboard and one that supports Xfire :hmmm:. Well I hope you don't mind spending money because they don't come cheap (tell a lie there is one, I'll come back to that one), the first motherboard I would put forward is form Intel themselves, its the
'Intel Extreme D975bx-305 975X' aka Bad Axe. Years ago no self respecting system builder would use an Intel own brand motherboard, but this motherboard is arguably the best 775 motherboard on the market.
Another board to consider is the
Asus P5W DH Deluxe
(scroll down) , supports Xfire, a decent overclocker and comes with WIFI as well. It's also slightly cheaper then the Intel board coming in at around £150.
If you don't want to overclock you CPU and your looking to save a few a pounds but you want it to support Xfire then get the
Asrock ConroeXFire-eSATA2 (first one). It only costs £63 which is less then half price of the Asus board and Intel board. But before you get to excited (after all this is a tempting option giving how much money you will save) this board is based on the Intel 945 chipset not the 975X and it only supports DDR2 667 Mhz. Although that doesn’t sound bad it is not the worst of it, don't expect top notch Xfire support the 2nd PCI-E graphics slot just sports 4 lanes (half of what you actually need and a quarter of what you actually want). In case you’re wondering as far as overclocking goes CPU voltage, CPU multiplier and Ram timings have done a disappearing act from the BIOS.
On your travels you might come across the
MSI 975X Platinum PowerUP Edition, it supports Xfire and is a good overclocker, loads of features and is less money then the Asus and Intel boards. However I hear a lot of people are having troubles with this board trying to get it work correctly. Potentially it's only a BIOS flash from greatness but you might want to do some research on this board before even considering it.
BTW if you can do without XFire buy the
Gigabyte GA_965P_DS3.