@ everyone who've replied on this thread. I have noticed that everyone here is just concerned about IP address while hosting multiple domains over a single hosting account. Though I am also not a SEO expert.. but over the course of time I have learnt that as a matter of fact - Search Engines do not rank websites..!!!! They rank web pages :) Thus for a given keyword SE bots / crawlers look for meta tags and scan the content of webpage. And the relation of the keyword to the available pages over the internet are indexed in the form of "Page Ranking" not the website ranking.
When we ask for result from a Search Engine by providing keywords then indexed / cached web pages are displayed in order of the page ranking in SE's result page, which is determined by the bots ascertaining the relevance. So to the extent of my knowledge page content is much more relevant and it over rules the aforesaid effect due to shared IP or different domain names. Though external linking is altogether a separate issue to be addressed while talking SEO which could even happen when domains are hosted on different hosting accounts. And I believe that hosting multiple domains on one hosting account is not bad at all indeed, rather cost effective. Corrections are warmly welcomed :)