I like to encourage people to get involved in OSS projects, so I'll take a stab at these.
1. If I participate in an Open Source Project and am a regular contributor, what impact does it make on the interviewers ? How would that help me?
In the normal case, neutral to slightly positive. Could be quite positive (if the interviewer happens to know and like the project), but it could also be negative (if the interviewer knows and dislikes the project). But in general it's a sign that you're capable of working in a software team, and that your work is appreciated by the other team members. Those are qualities that are otherwise hard to find out in an interview, but which interviewers are looking for.
Having said that, I don't think that looking good in an interview should be the primary motivation to join a project. You should be doing it for the love and fun of doing it, and prepare yourself for at least a medium-term, if not long-term, commitment.
2. How much time it would take to get in to the role of commiter and how much time per week I will have to keep contributing
That's really impossible to say. In general it'll take a lot of work before you're invited into a high-profile project (say, Linux Kernel, Apache httpd, ...), but it may be much less for smaller projects. Those usually go through phases of more and less activity anyway. And, being all volunteer efforts, it's understood that sometimes real life interferes.
Here's a project I encouraged that seems to have attracted a few additional developers already: XML DIFF Utility
Here's a high-profile one that's downright begging for help (and something that gets asked about in these forums every week - you'd be making a lot of fellow Ranchers happy): Apache POI (HWPF)
The only 'gotcha' I can think of would be an open source software (OSS) project that creates a conflict of interest with your current or prospective employers.
For example, if you're a known contributor to the Linux kernel, you might have a difficult time getting hired by a commercial Unix vendor who doesn't want to risk introducing GPLed code into their product and doesn't want to see code from their product going into a competing OSS project.
Also keep in mind that some non-disclosure statements could bar you from working on some OSS projects and/or jeopardize the legal standing of the project if you were to get your code committed to it.