GPS is going to be a hit and miss in India. Many times, you cannot look at the road and determine which way the traffic is supposed to flow. Constructions are announced at a fairly brisk pace, and get completed in the contractor's leisure. The end result is that traffic gets rerouted fairly regularly, and no gps is usually good enough to catch up with all this drama. Add to it that every state has their RTO and none of them are centrally linked.
Long story short, the best on-screen navigation solution that I would recommend is google maps on android. Requires a data plan and a working cell connection, but thats a given in 90% of the places you might visit. If you insist on an offline solution, I've heard good things about mapmyindia.com. I haven't used it, but two of my friends are happy in their cars.
Of course, the best solution is paper maps. As was pointed out earlier, the better solution is eicher maps. The one that I used to own looks like this: http://www.flipkart.com/india-road-a...=9789380262376
, but i remember it being less pricey. Another thing that always works for me is to stop at a tea shop, and ask for directions. It's very 20th century, but works most of the time
Good luck, and do post a RR when you head out.