Judging by TweakTown's analysis of the demo, if you want to push the resolution to 1280 x 1024 or higher, you're going to need more grunt than what the mid-range GeForce 8600 and Radeon 2600 can offer.
It's all good news for GeForce 8800 and Radeon 2900 owners, with these cards scoring in the 30-40fps range with all settings on high, and ~60fps on medium detail. Not a bad showing from the 8800, when you consider the chip is a year old.
Note that these benchmarks were performed on a system with a quad-core CPU, so you might be hard pressed to replicate the results on a dual-core.
Crysis Demo Performance Analysis [TweakTown]