One of my site is well ranked in Google, now I am going to change the host and it will change the IP address of the site (but same domain name). Will this effect my ranking in Google?

No, as long as the change is seemless there won't be any negative effects.

agreed, it will not hurt unless the site goes down.

well, it's not sure, more like a rumor, but the location of your host could influence your results; higher ranking in google.co.UK if your site is hosted on a UK server, otherwise, no changes what so ever

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These are some tips that Matt Cutts suggested to make sure that the move to a new hosting doesn't affect your rankings:
http://www.mattcutts.com/blog/moving-to-a-new-web-host/

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Moving to new web host will not affect your ranking unless you have multiple sites on same topic at on same IP and are interlinked

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