Originally Posted by Tetras
The P35 and P45 chipsets are both targetted at the "mainstream", while X38 and X48 are both "extreme", targetted at enthusiasts.
The most significant difference between P35 and P45 is that P45 uses 65nm technology (can potentially run cooler and use less power though with the added features it doesn't always work that way) and has PCI-Express 2.0 support (along with more lanes).
The most significant difference between mainstream and enthusiast chipsets are the number of PCI-Express lanes available. P35 can offer Crossfire mode working in a 16X/4X configuration, P45 in 8X/8X (though being PCI-Express 2.0 bandwidth wise this is equivalent to 16X/16X if it were the PCI-E 1.1 slots of P35), X38 16X/16X (PCI-E 2.0 so 32X/32X to the P35 PCI-E 1.1 slots) and X48 the same as X38 (the X38 and X48 chipsets are identical on a technical level).
You'll also find that most P35 and X38 boards are DDR2 but all chipsets (P35/X38/X48/P45 can support DDR3 should the board manufacturer choose to).
Officially there is also a difference in memory and front-side-bus (FSB) support but in practice you'll find the manufacturers that use the chipsets consider them equivalent in this regard (for example, X48 has official 1600 FSB CPU support where X38 does not but Asus advertises and supports their X38 motherboards with 1600 FSB).
You'll also find the new southbridge (10) is only offered on later boards (mostly P45) but it is mainly just a die-shrink so I wouldn't worry about this.
In CPU support all of those chipsets have official support for 45nm CPUs but in practice many P35 and X38 boards require a BIOS update to identify them (but may boot without issue, declaring the CPU as "Unknown"). P45 and X48 chipset boards are modern enough the BIOS can often recognise the CPU straight out of the box.
If you go to the Intel website and click on the block diagram for each chipset you'll get a fuller picture (the specs are publicly available but note that implementation of features in a chipset depends on the manufacturer, just because the chipset can offer 6 SATA ports or 3 PCI-Express slots doesn't mean the board manufacturer will make use of them all):
http://www.intel.com/products/deskto...8-overview.htm