you'll need to update your I-20, visa doesn't matter since it's required ONLY to enter the States. I-20 is what says that you are in status and allows you to stay in the States. and an I-20 expires either when you reach the expiration date stated on it or when you finish the degree stated on the I-20, whichever happens earlier. no matter what, your current I-20 is going to expire the day you graduate. in order to maintain status,  you'll have to either get an OPT, or get another I-20 from your gradschool. i don't recommend you to leave the States without I-20 from your gradschool. whether you get an OPT or a new I-20 from gradschool, don't stay in Nepal for more than 5 months, that's the max time you can stay outside of US without being out of status. 
i see a problem for you - if you get OPT, you'll have to work which means you can't stay in Nepal for 5 months; and it might be hard to get I-20 from gradschool since there'll still be 6-7 months before you actually start gradschool. if i were you, i'd apply for OPT right NOW, work for 4-5 months or so; get I-20 from gradschool sometime in late june/early july and go home for a month or two and come back for gradschool.