Short version...
The trick is to treat the ENTIRE yard with a good product, both of what stevek mentioned are good. But first, use a fire ant bait that's specifically meant for ants and not mixed with weed killer or fertilizer. Also, don't disturb the mound, because they'll send out their soldiers and the queen will go into hiding. After treating the yard (or areas you want) let them go on with their daily routine and before ya know it...they're gone.
Then if ya want to use other products that are mixed, go ahead..the bulk of the ants should be gone and the mixed product will be more of a maintenance
against fire ants.
~Ray