GTA IV > Crysis in terms of demandingness, and arguably in Gameplay too..
Crysis wins on looks, but GTA IV's engine was worse than Crytek's.. they just did a bad port (sorry Take-Two but that's really what it was)
You need a decent Quad-core and at least an 8800GT 512MB (the 256MB gives you that RAM Overflow issue) to run @ 1280X1024 decently (and its still at 30FPS)
^^ Yep, it shows I had 1GB of Graphic support and at like 50% of all detail settings it was filled to "Yellow" which meant I was pushing my system. I was like WTF? A very demanding game, but definitely not as pretty as Crysis.