Since you're already on 3.1.3 you don't have to worry about SHSH blobs. You can always restore to 3.1.3 nice and easy using iTunes without any modifications.
Once you jailbreak, open up Cydia and make sure you tap "Make My Life Easier". After that you never have to worry about manually getting your SHSH blobs, Cydia does it all automagically.
It's great that you've done the research, now dive on in. We're here if you have questions or problems
