P35 if you want to go SLI. X38 if you want to go crossy. The x38 chipset is alot newer and supports some newer tech but, the only reason i would go with x38 is if i planned on crossy.
I think x48 has only ddr3 support, as in DDR3 speeds only.
also depending on the motherboard, x38 and x48 will support the 1600FSB cpus in the future, good for if your looking for a longer investment.