Thanks to plethora for inform me that I840 and I820 datasheets are up on Intel's site. The other interesting thing is that Intel changed the Coppermine 677 and 733 benchmarks from i840 platform to i820 now. The biggest thing one can see is that SPECFP95 scores dropped......... the performance went down from 30.4 to 28.1on the coppermine 733 by using i820 instead of i840. For reference i saved both Intel's i840 html and PDF locally since intel doesn't link to to them anymore i have a feeling they are going to yank it for fun. Although SPECINT95 did went up from 35.6 to 35.7 on i820, and highend winstone also went up from 39.8 to 41.1.
Now going to i840's datasheet one can see that it have a 8 deep in-order que for mmeory requests as opposed to i820's 6. Although Intel i820 is considerablely cheaper to package at 324 pin compared to BX's 492pin or i840's 544pin. (the fewer pins the cheaper the computers can be made) The other measures to save pin cost for i820 include support for only 32bit addressing (no 36bit here) and support only single CPU platforms. i840 also allows better PCI performance by offering a high speed Hub interface with twice the bandwidth of the ones found on a i810 or i820. For a local copy of i840 performance PDF, or html. Also check out the designer's guide for how low the noises have to be and trace requirements on i820 and i840.