I say go ATi currently. I've been both an ATi and nVidia fanboy in the past, but I've come to the conclusion that neither one is the hands-down winner. ATi has their good moments just as nVidia does, but right now ATi is winning. The 58xx cards are more power efficient than the GTX4xx cards while providing about the same performance. I prefer the ATi heatsink design and ATi provides more display connections (allowing up to 3 monitors on a normal edition card which can all be used together for gaming in Eyefinity).
The ATi equivalent to the GTX470 is the HD5870. Like you, I had my heart set on a new GTX470 but was talked into ATi after posting my build here. After reading plenty of reviews I decided that the 5870 actually was the better card and bought one. I'm quite pleased with it, the graphics performance is excellent and the idea of having 3 monitors was too tempting so I ended up buying 3 monitors and Eyefinity is just plain awesome.
The only reasons to use nVidia these days are CUDA, PhysX, and 3D. CUDA is nVidia's proprietary general-purpose GPU computing library, but ATi supports OpenCL and DirectCompute, both of which are standardized competitors to nVidia's proprietary CUDA, ATi and nVidia both support OpenCL and DirectCompute with the newest drivers. PhysX is an nVidia technology based on CUDA that allows the GPU to process intensive physics simulations. It is implemented in several games but can only be used in those supported games, so it is fairly limited and will hopefully be replaced by an OpenCL or DirectCompute based physics library in the future. The other nVidia "killer app" is 3D. nVidia supports 3D rendering using their 3D vision glasses and a 120Hz monitor. This setup may be expensive but it's nVidia's answer for Eyefinity.
