Vaibhav,
Sorry to suggest something.
If you have a 4+ inches android phone, then install sygic /mapmyindia from android market (which gives updated maps free even software). Many here using there phone as GPS device and most of them are happy about. You just need a mobile car holder for that. If you do not want to use your mobile, you can even think of buying a dedicated 5" phone (which got gps) and you can use it.
Advantages : You get much better hardware than the 300mhz processor
You can try other navigation software if you require.
You can use as a phone a later stage when you are not using in car.
You can pair with bluetooth device and stream music from your phone to headunit (you can use an aux cable too)
Even the thread starter is using Sygic and he is happy and never purchased a dedicated device.
If you are purchasing a dedicated device, go for a bigger display. A 5" screen so that it does not distract you much .
(Just a suggestion. Rest is your choice)