BUFFs link has all the info you need but in short (I've missed a few but covered the Intel chipsets you are most likely to see on sale):
Core 2 Generation 1
975X: Not designed for Core 2 but the old Intel "high end" chipset, still as fast as any clock for clock but does not clock well with Quads and any support for 45nm chips is unofficial (though BIOS support for Wolfdale is not uncommon).
Core 2 Generation 2
P965: First mainstream Core 2 chipset, PCI-Express is limited to 1 16X slot and 1 4X slot. Clocks pretty well and may support 45nm CPUs but has been made obsolete by P35.
Core 2 Generation 3
P35: 16X and 4X PCI-Express just the same as P965, but clocks very well and is now a mature chipset (was always stable) with official support for 1333FSB and 45nm CPUs.
X38: First board with PCI-Express 2.0 and is the replacement for 975X. It supports (I think) 2x PCI-E 16X slots (PCI-E 2.0) and 1x 4X slot (PCI-E 1.1). It clocks similar to P35 and officially supports 45nm CPUs just the same.
X48: Adds official support for 1600 FSB CPUs.
