[u]TI[/u] makes lots of power amp chips. For more power, you'll also need more voltage. I assume you have more voltage available in your robot.)
You may need more voltage - Power is calculated* as V2/R. That's RMS voltage and with a "normal" (single-ended) amplifier you get a theoretical peak-to-peak voltage equal to the power supply. (In practice, you can't get the full power supply voltage swing.) The RMS voltage is about 0.35 x the peak-to-peak voltage.
So, with 5V and 4 Ohms you can get about 3/4ths of a Watt.
With a bridge design you double the voltage for 4 times the power, or about 3W. (The PAM appears to be a bridge amplifier.)
- That's one way to calculate power... The basic power formula is Current x Voltage. The other formulas can be derived using Ohm's Law.