sli questions

okay im gonna sli my 250 with another one, now i know this one since its faster than the new one in most areas itll down clock, there my problem, the new ones memory clock is faster than my current one does the shader and core clocks get downclocked while the memory clock gets over clocked?

I believe mixed clocks only run at the slowest speed to keep everything compatible. So everything gets down-clocked.
This will make you an expert on the subject:
http://www.slizone.com/page/slizone_faq.html