1. Let's break this down:
GABA receptors are inhibitory neurotransmitters

GABA(a) transports Cl- into or out of the cell (the direction depends on the Cl- concentration)
GABA(b) transports K+ into or out of the cell (the direction depends on the K+ concentrations, typically potassium is higher inside the cell)

The resting potential of the cell is -70mV on the inside (meaning there are more negative charges inside the cell; the outside potential is 70mV). When GABA(b) is selectively activated it drives the export of K+ out of the cell inhibiting depolarization. The net result is that the cell becomes even more negatively charged (think about it the inside is -70mV and then you activate GABA(b) which kicks out even more K+). The level of inhibition will depend on the amount on the length of the signaling event. Check out http://www.biology.eku.edu/RITCHISO/301notes2.htm for a really great image of this towards the bottom of the page.

2. First let's make sure we know the structure of the neuron (see http://en.wikipedia.org/wiki/Neurons). The dendrites are the signal receiving branches of the cell, the soma is the cell body where the nucleus is located and protein translation occurs and the axon is a long projection that ends in another branched tree of axonal tips that are the transmiting part of the neuron. So the dendrites receive the signal and the axon release neurotransmiters to affect the membrane potential of the next neuron. The dendrites can also signal to adjacent neurons. Because of the specialization in structure the dendrites would be more effective in evoking action potentials because of the large amounts of receptors and ion channels, gates and pumps located in its membranes. The function of the soma is to produce proteins and the amount of transporters on its membrane that could receive a signal and modulate it (such as amplifying it) would be minimal if any. See http://en.wikipedia.org/wiki/Dendrite and http://en.wikipedia.org/wiki/Soma for a comparison of the structures.