^Yeah, tolerance builds rapidly and people end up taking huge doses, even legitimately prescribed doses of 1200-1600mg aren't uncommon.
But yes, to answer your question, start with 300mg and see how you feel. I recall the first time I took it, that amount was enough to get a feel for the drug. Many people don't like Gabapentin, but I'd say if you are going to take it, you'll probably enjoy it a lot more if you take it with a Vicodin or two. Xanax releases GABA much in the same way that the Gabapentin does, so maybe hold off on that for now.