Summary: First of all, the X1950 XTX is not such a big upgrade from an X1900 when it comes to performance. This is more of a product refresh where existing weak points have been addressed. Most important, the new cooling system which offers quieter operation and lower temperatures.
The other major change is the move to GDDR4 which runs cooler and faster. Being able to run 1000 MHz real memory clock gives a nice performance boost. About 8% in the benchmarks.
Pros: Competitive price, Quieter Operation, Performance improved, Lower power consumption, Works with X1900 Crossfire Master, DDR4 should offer nice overclocking headroom
Cons: Paper launch, Performance increase is small, Overclocking is not fun