A 9800GT would beat the R4670 by a margin. The R4670 is equivilent to a 9600GT. The question AMD or Intel? Well that depends on what you have. If you compare a Core 2 duo and an Athlon X2, the Core 2 duo will smoke the Athlon x2. Intel would be overall better because the Core 2 duo can overclock high and beat the Athlon x2's in many benchmarks. But if you get a Phenom II then AMD is more comparable to Intel in terms of performace.
I agree with the 4830 over the 9800 GT, but don't think it dominates the 9800 GT. It does come out ontop in alot of the tests, but ofc like most cards in the same category, they exchange blows sometimes.
the 4830 is a better value, but the 9800 gt is better (slightly).. they are both great cards though when paired with a dual core 2.6 ghz or higher. you will get good performance with any game at good resolutions.