the X1900 has 16 pipelines that are supposed to do 3 times the work as their previous cards. but there isn't all that much difference between it and the X1800.
right now, the X1900 is the fastest card on the market. however it won't be for long though. Nvidia has yet to release 2 cards
personally I'd say wait a bit for at least the G71 (Geforce 7900, should be released pretty shortly) or if you're patient, wait for the G80 (Geforce 8800, should be released in 2-4 months)