The smaller die is NVIDIA's High Speed Interconnect (HSI) which have have recently seen on the NV37 at Computex. Basically the other die is just an AGP-to-PCI Express bridge, meaning that NV45 is still an AGP part.
Our good friends at ATI have informed us that the HSI chip causes more heat and that the NV45 is having problems running at full clock speeds let alone being overclocked. I guess here in the short future I will be able to tell what I say my thoughts on heat and performance without being gagged with an NDA.
The NV45 or Geforce 6800 GT Core:
Looks like the same HSI as the NV37 we reported on from Computex:
