Crysis is designed to be first and foremost an engine for licensing purposes. This explains why the engine was available for sale well before the game is even ready.
Additionally, Crysis (according to their website and in the unofficial crysis-online page) have stated the game will be designed to scale nicely depending on the system. They have targeted computers that are two years old (from time of release) to 1.5 years in the future. Not 10 years! This approach is very common in PC games, including Doom3 (when it came out, its "Ultra" setting was designed for hardware that wasn't available at its release, but somehow nobody complained then, and you look at the game now!).
As for Crytek disabling features that your computer couldn't handle - what's wrong with that? If anything, that means you won't be able to enable settings that will cause your computer to crash, and that's a good thing!
There is no reason why Crysis cannot be ported to any console; it's the amount of detail and/or resolution that may be dumbed down. Just look at Far Cry on Wii - crap at best, but yes, technically, it can be done. Given how the PS3 and 360 are vastly more powerful than a Wii, I don't see why Crysis cannot be ported (albeit with numerous features dumbed down or turned off completely). Crytek have retracted on their word that the game cannot be ported to consoles. If anything, PC games are the ones best suited for porting, as they are always designed and coded with various hardware in mind to begin with.
Finally, Crysis is designed to be a DirectX10 which downscales to DirectX9. By definition, DirectX10 is available only on Vista, which is a 64-bit system, so you will certainly benefit from having 4GB of RAM; XP is 32-bit only so 2GB is optimal, but in any case, Crysis will run in XP, just without all the nifty shader effects and other graphic niceties that DirectX10 brings to the table.
