I see both sides of the issue. First, Dual X1950 pro's now would not be as good as an 8800GTS. It doesn't sound like the 8800's are even on your list though.
Spending $400 now on an 8800GTS may not be feasible/worth-while for you. Seeing how no master card is needed, you may find a cheap X1950 pro someday and say hey, I'll go for crossfire.
Oblivion for example is a game where crossfire X1950 pro shines, easily beating out a single X1950XTX.
The third situation that I think many of us would just throw out there is that come the day you may want to get that second card, you'll be looking at a much faster mid-upper range DX10 card instead. Look at crossfire X850XT. Anyone who once bought a master card thinking they will pick up a second card later, probably instead went with a single X1800XT or X1900XT when the single X850XT wasn't cutting it. You also have to worry about supply and pricing down the road.
Anyway, the x1950 pro if priced right is a good card now, so I see no issue with your plan. Whether you ened up with a second X1950 pro, or just a different card later is something to decide at the time you aren't happy with the X1950 pro. An X1950XT for $220 or so would be better if priced close and 8800GTS better than that, but it's your money and your gameplay that has to set the limit. Just don't spend an X1950XT amount of cash on a X1950 pro.
Personally, I never considered crossfire as an option but saw my last upgrade as the card that will take me all the way until I want a DX10 card. But it will take the release of R600 and the game crysis before I'd pick what that card would be, be it NV or ATI.