According to forbes, the amount is estimated to be about $50,000,000,000 ... yes billion. Microsoft will own everything Yahoo... technology, networks, trademarks, etc... I think that combining both of their technology and unlimited resources as separate search giants... could be a serious threat to google. It won't be a one combined name or brand... it will stay the same.
If MSN buy Yahoo , of course there is a change of name to Micro-Yahoo. And I think there is changes in the algorithm and as well as the whole business and services.
Those things never gonna change yahoo remains yahoo and microsoft remails microsoft ,the only thing is they will work together.
Hopefully if microsoft buy yahoo there will be a merging of technologies to make both search engines more powerfull. I personally look forward to it because all of my sites perform better in yahoo and MSN.
theyd probably keep the two names , msn might iron out some wrinkles in yahoo , like having yahoo photos & yahoo flickr as well , two products doing more or less same thing , etc.