Thats an interesting way of choosing a graphics card, never thought about it like that before!
I'll be getting a P180 too, and am going to attempt to cram water cooling into it, so if you're interested I'll let you know how it goes.
As far as future lastability goes, its a tough one to call right now, as you probably know DX10 cards are just round the corner, but if rumours are to be believed the G80 (nvidia) will only dedicate 1/3 of its proccesing power to DX10 (as it will have dedicated DX9 and dedicated DX10 pipelines that cannot be changed between the two) so will not have very much power in DX10 games (but will have DX9 speed). While the ATI card (R600) will have something called Unified shader architecture which means that the shaders can be programmed and will be able to switch from DX10 or DX9 use, so should be alot faster than the G80 in DX10 (and possibly DX9), but wont be released until january.
Also both these cards are rumoured to be consuming 175W (G80) and 200W (R600) of power per card, if thats a concern then get the best DX9 card you can now and wait to see how it pans out (The 7900GTX is still more 'power effcient' compared to the x1900xtx/x1950xtx.)
Back to your original question; right now if you want pure speed, go for the x1950xtx. If cost is an issue the cheapest of the 3 will do just fine (not sure which it is right now). If you decide to get the x1900xtx, you will want an aftermarket cooler (air or water), as the original is very loud. As far as performance differences go, check out the recent article on legitreviews of the x1950xtx.
Also what resolution do you plan to be gaming at?
I know its alot to take in, but there are alot of factors right now
Dan
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