Shall I open PPF account now or after 3 years?

Dear all,

My topmost goal is to buy a flat within 3-4 years.For this i am saving money through RD & FDs only.

I want to open PPF account,i am in dilemma

1. to open PPF account now & deposit Rs500  to keep it alive & after purchasing flat start actual investing in it

OR

2. open PPF account after purchasing flat

pls suggest me.

thanks in advance!