Help: Query regarding academics!



I am an undergraduate student completing my masters(its bachelor+master) in Mathematics & Scientific Computing.I am a student of IIT which is considered to be Ivy league school in India.You need to clear a competitive exam where top 1% are selected put pf approx. 3,00,000 appearing.I want to go for an investment banking job.I have a very good 10th(secondary school) & 10+2 grades(higher secondary).You can say its equivalent to having A in all subjects comparing to UK standard.But then i screwed up my grades (GPA) in my undergraduate studies .Its around 6/10.Earlier i never knew grades could have so much impact on future career prospect.As i see here its all about grades ,grades & grades.May be naivety or my complacency of reaching till this far lead me to this situation.I wasted my time while LAN gaming and sleeping.I did some extra co-curricular.I would like to get some advice on how to go for my future career as i will sitting for placements next year.What arguments should i use to defend my bad grades? Can i still be able to make in any investment baking? or Should i go for a MBA as to improve my resume?(here you can go for MBA without work ex. from good Indian B-schools).Earlier i didn’t have any plan of getting MBA so early.But now chances of me landing in some mediocre job is very high in such credit crunch.So i am confused a lot.

All further suggestions will be really appreciated.