I will answer
your question without adding anything to it
(without overclocking)
Both i810 and i815 can run up to 133MHz FSB.
i810 can only run up to 100MHz memory
i815 can run up to 133MHz memory
If you put your Celeron in the i810, you would have to run it at 100MHz FSB and 100MHz memory.
If you put your Celeron in the i815, you should be able to run it at 100MHz FSB and 133MHz memory.
Hope that clears things up.