First, the X1900XT is a huge card, so make sure you have enough room (both length of motherboard and nothing next to the card as the cooler is big).
Second, the X1900XT has 16 pipelines, but 3x the pixel shaders, so 48 effective.
Third, go with the X1900XT. SLI can be hassling even to the pros. 2 cards will make more noise and generate more heat than a single card. If what people are saying is true, the X1900 series, both the XT and XTX, have a very nice headroom in terms of overclocking (assuming you're into that oc department thing at all).