the 965 chipset is the mainstream choice for core2duo with good performance in a moderate price motherboards. the intel 975 and nvidia 600 chipset is higher end and in more expensive motherboards.

the asus p5b line is probably the most popular 965 based motherboard line. the biostar tforce 965pt is slightly cheaper and a good all-around performer.

http://www.anandtech.com/mb/showdoc.aspx?i=2914&p=1