I can't speak towards WiFi issues but many times you simply cannot upgrade from Android 1.5 to Android 2.x. Why? Because the tablet hardware simply cannot handle the new and more complex operating system. In some cases the Chinese manufacturers that built these tablets used the specific versions of Android that they did because it was what was available that would run on the tablet.
If you want 2.x support on a knock-off tablet you might find yourself doing some/much of the development/testing work to get it to run on that tablet. Otherwise, if you are lucky, maybe one of the techies over at XDA will do (at least some of) the development and testing work for you.