andreasl: wouldn't work I'm afraid.
[*]Firstly the RIMM riser was an Asus proprietary thing, everyone else just had it built right in. Though I suppose it might work with other boards...lets assume it does...
[*]Secondly the MTH chip was only for i820, even the i840 had a different chip to perform that same function (MRH-S). Though I suppose i840 and i850 have similar memory controller, so lets pretend for a minute that the i840 MRH-S would work with the i850...
[*]Finally, lets pretend that the i840's MRH-S would work in an i850 board and you could get a riser card with the MRH-S on it that would fit in an i850. There's still the fact that intel recalled any motherboards with MTHs on them, and issues RDRAM based i820s.
I see where you are coming from it would be neat to see, but I think it's very unlikely.