Radioactive methods are more sensitive and I believe, it would be your only choice if an ATPase activity of monomeric actin to be measured. On the other hand, ATPase activity of actin upon polymerization can be reliably detected by EnzCheck Phosphate assay kit from Invitrogen. This method is reliable in the range of low micromolar Pi concentrations, but not much below that.
If you can purify your protein, spectro-fluorimetry and kinetics assays with fluorescent of actin bound to ATP analogs, whose properties vary upon hydrolysis of the game P, allow you to follow its ATPase activity in real time.