even if you got the 1950xt as far as my knowledge it would only be as fast as your second card, therefore proving pointless to even get the 1950xt
The cards do not run at the same speeds, despite early speculatiuon and restrictions to the contrary. Both cards will run asynchronously at their max. The main thing is that they have similar design, and most importantly that ATi enables support for that combination in their drivers.
The X1900 and X1800 couldn't be paired due to major fundamental differences in their design, but the X1900 and X1950 would be similar in architecture only different in clock speeds (and ram but that's unlikely to alter things).
I think it would be wise for ATi to enable X1900+X1950 crossfire, but that doesn't mean it will happen, as many previous logical and benificial things have not occured for one reason or another (usually marketing though).
Anywhoo, X1950+X1900 will both operate at their stock speeds, however if there were a scale difference like 6 equally functional groups versus 4 equally functional groups, then the second card would not use the 2 additional functional groups in Xfire. Even this shouldn't be an issue technically, but with the way ATi have implemented it this is the way it currently works.
edited for my bad quick dyslexic math. 6-4=4 right?