The rule of thumb:
Never, EVER buy new hardware until there is something you want to do RIGHT NOW, but can't.
If all the games you play run just fine on your 4200, don't upgrade. If you want to upgrade for Half-Life 2 or Doom3, buy the video card when they are released... because hardware will be cheaper and better then.
But if there are games you're playing now, and the framerates are less than you find playable, the $200 range will get you a 9700 non-pro (if you can find 'em, going fast) or even a 9800 non-pro. (unless it has an "SE" in the name. Stay away from Ati cards with SE!!!!)
The 9600XT looks to be a great card, but either of these (9700 or 9800) will be better than a 9600XT.
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Radeon 9500 (hardmodded to PRO, o/c to 322/322)
AMD AthlonXP 2400+ (o/c to 2600+ with 143 fsb)
3dMark03: 4055