Google page rank has no relationship with the actual rankings i.e SERPS of any website. Read http://blog.seohawk.com/page-rank-vs-trust-rank/ for more information.
1) There is no such thing as "PageRank update". Real PR is ongoing process, your PR score is computed and used continuously. 2) What you call "PageRank update" is actually PR toolbar update. That means sometimes (usually once in 3-4 months) Google takes their internal PR data and makes it public via PR toolbar. 3) Real PageRank updates (which are continuous but invisible) most likely have remarkable effect to your rankings. 4) Toolbar PR updates (that`s what you call "PageRank update") have no relationship to the actual rankings.
I noticed that after the previous update, there's actually a boost in the ranking and traffic. It does make a difference.