If your sticking with Socket 775, your choices will be a bit limited with DDR3 capability.
For socket 775/ DDR3/ and crossfire support, you best option will likely be a X48 or X38 Chipset.
Sadly, your choices are going to be rather limited for DDr3 & Socket 775.
Maybe something Like:
ASUS Newegg.com - ASUS P5E3 PRO LGA 775 Intel X48 ATX Intel Motherboard - Intel Motherboards
DFI Newegg.com - DFI LANPARTY LT X48-T3RS LGA 775 Intel X48 ATX Intel Motherboard - Intel Motherboards
DFI are excellent boards. but a word of warning. If you plan on OCing, they are not at all beginer friendly.
They do however, have excellent bios, with a incredible amount of options. So unless you want to get really intimate with the bios, DFI may not be a great choice.
It really comes down to how much experience you have, or how much you want to learn all the ins and outs.
With socket 775, it might possibly be a better route to stay with DDR2, (IF you already have good DDR2 memory) simply due to the limited choices of DDR3/775 boards.