I personally dont think that it is worth it, if i had $500 to spend and i needed a video card upgrade i'd look towards the 7900GT (im seeing some for as low as $240 now) and use the extra money towards other things.
Btw if you wanna get a 7800GT/X just get the 7900GT, it outperforms the 7800GT and is about equivalent to the 7800GTX except the 7900GT produces less heat and consumes less power and also cheaper.
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