If you are planning on getting the card we mention, then I suggest waiting for the 4770. It will be the same price as the 4830, but slightly more powerful. Plus, it consumes less power.
The cards in each tier are not exactly the same, each will be a bit better in game X but less good in game Y but overall are comparable.
Nvidia seem to be a little cooler/quieter, ATI does AA and HD playback better.