Which is the best mainstream gaming GPU?

I'm planning to buy a graphic card, but I'm really confused on which one to buy. I've come up with two choices: the Nvidia GeForce 8600 GT and the AMD ATI Radeon HD 4670.

I'm planning to play games like Farcry 2, Fallout 3, Call of Duty 4: Modern Warfare, Crysis: Warhead and GTA IV.

My PC's an Acer desktop with a 2.2GHz Intel Core 2 Duo 2.2GHz, 1GB of DDR 2 Ram and a PCIe x16 slot. The PSU's 355W. I have an LCD monitor with VGA input only.

Which of my two choices is best? Or should I look at an alternative?