at what level do you want your condition to be verified? At the total level or at the detailed level? If you wanted to verify your condition at the detailed level, then the correct syntax is:
TextCount( If(A >= B, C) )
Otherwise, your condition can only be satisfied if only one value of A and only one value of B is available at the time of the calculation (which probably isn't the case).
Check out my new book QlikView Your Business - in the book, I'm describing in detail how to build QlikView expressions, starting from the very basics (like at what level of details should the calculation happen) and going into very complex expressions with AGGR and Set Analysis