I observed strange sudden spike of the Page/Sec value, what should I look at next and what other counters should I collect? Would appreciate any advice.

I'd run process explorer, http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx and identify processes that are using a lot of memory. Since this is being reflected in pages/sec, it would seem that one or more processes are hitting the paged pool really heavily. -- Mike Burr

A sudden spike could be anything and most likely this is not something to worry about. IMHO this is not a good counter to monitor because it fluctuates some much. If you had a base line to compare it against, and could give us actual numbers for the baseline and the "spike" that would help.