Dynamic function creation

=SUMPRODUCT(('Sheet1'!B$1:B$100=$B50)*('Sheet1'!I$1:I$100))

Sheet1 is sheet name. What I want is to have this name in a cell (say A1) and then use that text in above formula. i.e. something like...

=SUMPRODUCT((A1!B$1:B$100=B1)*(A1!I1:I100))

What function I use such that the text gets evaulated to formula parameters at run time.

Have you tried the INDIRECT function?