What is the difference between these chipsets X38/48 and P43/45

The x38/48 are both the same chipset with the x48 variants being rated for a higher FSB.
In practice, there is no difference between the two.
It is Intel's last enthusiast chipset for the Core 2 arch and offers 2x PCIe 2.0 16x slots running at full speed.

The P45 is Intel's last mainstream chipset for the Core 2 arch.
As it is produced at 65nm (vs 90nm for the x38/48) it draws less power and, on a good board, is currently the best overclocker.
It offers similar features to the x38/48 with an updated southbridge, ICH10 vs ICH9, and 2x PCIe 16x slots.
The biggest difference is how the PCIe slots are configured.
You can either run one at full 16x speeds or both at 8x each.

The P43 is a dumbed down P45, generally on lower end boards.
It offers nearly the same feature set as a full fledged P45 but you can not split the 16 PCIe 2.0 lanes into two slots.
As it is a value orientated chipset, it may not overclock as good as a P45 or x38/48 although most do.

NONE of these motherboards support SLI.
If you want to SLI 2 or more nVidia GPU's you will need a nVidia 700 series motherboard.
I would recommend against this as nVidia boards are not very good.
You can, however, run CrossFire on the x38/48 and most P45 based motherboards.
The difference between SLI and CrossFire being SLI is multi nVidia's only and CrossFire is multi ATI GPU's only.

It's on wiki already...