I think you confuse 'designing a game to be unplaybable' and designing a game to max out current and challenge future hardware.
I don't think Crytek is going to make Crysis a game that gets manhandled by a GF8800 or HD2900, if anything it'll be the other way around.
It's just a guess on my part, but looking at the early sandbox demo, I'm certain that you will have folliage and particles turned down and many visuals (like motion blur) turned down for the current high end cards.
That's not to say they will suck, but you're not going to be able to play with all the sliders pushed to the max. Looking at cytek's interviews that's pretty much what they also say where even SLi and Xfire rigs will be hardpressed to play at high levels. The game is aimed to be 'playable' under a wide variety of conditions, but it's unlikely that these settings will be maxed in the same way we've come to expect 'maxed' sofar in the last of the DX9 titles.
Considering just about every gameplay demo sofar was played at medium resolution, I doubt many people will be playing Crysis @ 16x12 or above with any heavy level of AA with the sliders turned all the way up. I suspect particle effects and foliage will just about kill any card when they are turned on high.
Your advice of letting the system select settings and go with it is likely what we'll all be doing, but I expect bad news for anyone expecting to see the words MAX or Ultra in those defaults. But personally if my hardware at the time just makes the game look somewhat similar to what they intended and look 3/4 as good as the demos, I'll be more than happy.
Like we've said before we'll have to wait and see, but don't think that Crytek is going to make this easy on new systems. It'll be playable to a wide variety of systems, likely all the way down to the X800/GF6800 series and maybe beyond, but it's different making it accesible (like the way HL2 was all the way down to an original radeon/geforce) versus allow options for future cards to exploit that are beyond the power of current systems (like FartCry, HL2, and D3 all did).
Just my two frames worth, but I wouldn't start wasting money on a GTX for an unreleased game on the gamble that it might be able to add something more than just 5 fps.