okay wait-- are you going in February? because your other thread says mid-march...
Goa can be fairly cheap, but it's mostly just a beach destination, not really a hotbed of current Indian culture. I don't mean that to sound snobby, Goa is fine if your plan is to go sit on a beach for a month-- with a LOT of hippies and ravers to keep you company and a few beautiful Portuguese cathedrals dotted here and there. Although personally I would recommend Kerala instead, it's farther to the south, more beautiful, and way less polluted. Heading to the beach is a good "break" from the hustle & bustle of the continent, for that reason I'd recommend going there either mid-trip or towards the end when you're burnt out on all The Crazy that you'll experience elsewhere.
Trains are dirt cheap and fabulous. On my first two trips I traveled either chair class or 3-tier AC (sleeper). On my last trip I traveled plain Sleeper class, which is basically almost exactly the same as 3-tier AC only there's no air conditioning and it's only about 1/3 of the price. Some routes can be excruciatingly long, my trip from Bangalore to Varkala, Kerala took about 21 hours.
Whatever you do, please don't do overnight bus journeys, or at least to keep them to a minimum!
The saying goes that "you are more likely to die on an Indian bus than in any other way". I can attest, having taken three overnight bus journeys and a few other claptrap ramshackle daytime ones, that this is undoubtedly true.
See my response to your other thread for more info, and please let us know more about your trip.