You can purchase and install the -20, 40 and 60 FirePOWER blades but cost-wise you'd be much better served by the newer and better price-performance of the 4100 series.
You need to analyze your functional requirements though because to get the FirePOWER services you'd need to run the FTD image. There's not yet feature parity even with the just-released 6.1 code.