the 7800GTX easily beats the X1800XT in most tests, but it is more expensive. I would go with a 7800GT/GTX instead of getting a 6xxx series card and SLI them.

As for the topic, i dont think that the X1800XT 512MB is worth it, the average score in 3DMark05 was around 9500 with that card, and you can get a much higher score with a 7800GTX.