when the x1800 series was released it didn't do too well due to poor drivers, but after a few months, drivers improved, and so did performance. the x1800xt was equivalent to the 7800gtx at that time.. and the 7900gt is equivalent to both.. in some cases one card pulls one way, in others, a different way.. so overall i'd say they're equal. so it all goes down to price. whatever is cheaper, i'd go for it.
x1800xl = 7800gt
x1800xt = 7800gtx/7900gt
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