Why is Nforce 5 a two chip design?
 

WHILE NVIDIA is still working hard on the Nforce 5 chipset, we have some things that might explain why Nvidia actually had to go for a two chip design with Nforce 5.
The first argument is that the Pentium chipset requires a memory controller as the P4 doesn't have memory controlled built in the die. That's unlike Athlon 64/FX CPUs

http://www.theinquirer.net/?article=20855