I think this is slightly pointless. The 9600 and 9800 are ATI's older line of graphics cards, so comparing them to nVidia's 6200 or 6800 series is pretty harsh.
Anyway, although nVidia had a hard time (In my opinion, failed. Especially in DirectX 9 comparisons) keeping up with ATI's 9500 through to the 9800XT through the use of the doomed GeForce FX (5th geforce series) they have came out and compleatly walked all over ATI with the 6200 through to the 6800 series of GPU's (keep in mind that the 6200 is in comparison with ATI's low range GPU's - the X300 and X600, while the 6800 is in direct competion with ATI's X800 and above).
Anyway, if you are concerned about cost, go for a geforce 6600GT. It is roughly twice as fast as a ATI 9800xt and currently ATI has no direct competion against it (although some regard the X700 as the competion against the 6600(GT) - where the 6800GT also wins here hands down).
Keep in mind that there is a pretty large difference between the 6600 and the 6600GT performance wise. the PC- Express version is also, by default, slightly higher clocked and results in a slight performance boost over the AGP version.
Oh, an added benfit also is that the Geforce 6 series of cards support the latest DirectX 9.C specifications and some likely future ones too, while the ATI X series of cards only support DirectX 9.(I think B - Definatly not as recent as the Geforce 6 series)