your dead wrong about crysis. Crysis runs in CryEngine2 which is the most complicated and resource hogging engine ever made and it can't even run on the consoles.
GTA IV was built on the new RAGE engine that rockstar games made for table tennis and then they used it to create gta.
as i've actually looked into stuff, this will most likely be the MINIMUM specs for a machine with vista and this is assuming no DX10
2.5Ghz dual core
8-12GB of HDD
Now, for max look for something along these lines
3.0Ghz Dual core or higher
4GB DDR2 Ram
512MB GDDR3 or higher video card
and that will be for like 1280x1024 with all settings on max, full AA and all that jazz. but either way the specs will be no where near what crysis needs for high/max