that's quite a bit of gain, if you put a 1 watt amp on those, you'll be at 251 watts EIRP, quite a load, and pretty dangerous. remeber you're working with microwave here. if you stick with just the WRT's output (28 mw last i remember) you'll be at 7 watts. that alone should be enough to get you that distance. IF any more power is needed, i;d suggest flashing those wrt's to ddwrt, that lets you select more output if you need it, with those antennae, and amp definitely isn't necessary.