the difference between
scjp 6 and ocpjp 7 is in the version of
java. both are programmer exam but the first exam i.e. scjp/ocpjp 6(1Z0-851) targets java 1.6. whereas the second exam ocpjp 7 is for java 7. both are programmer exam but they are different in respect to objectives and also if you go for latest version it would be more good.
to achieve ocpjp 7 you have 2 options
OPTION 1 : you need to pass OCAJ 7 associate exam and then only you can appear for OCPJP 7 exam
OPTION 2 : anybody who has SCJP/OCPJP certification(any version) can appear for upgrade exam for OCPJP 7.
both options has its pros and cons. you decide for yourself which will be the best for you
SCJP study guide by kathy seirra and bert bates targets SCJP/OCPJP 6. there is also latest version of book by the same authors for OCPJP 7 but i'm afraid it is not yet published. might be availabe somewhere around feb 2013. you need to check amazon for that.
regarding NIIT i can't help you there.
people have cleared ocpjp 7 with using SCJP study guide for java 6 and covering extra/new topics here and there from internet and here on javaranch.
Please check certification result forum for more details regarding exam preparation and methodology.