I've decided on the Core i5. I figured the $100 difference was better put into an SSD/graphics card/savings. According to various sources, some functions will see significant benefits from hyperthreading and others won't. But I figured the Core i5-2500K (overclocked to 4+ GHz) will already be a tremendous improvement over my current Core 2 Duo E8400.
I still haven't decided on the graphics card, mainly because I can't quite determine how much Photoshop will use it. Adobe recommends
Photoshop CS4 and CS5 leverage the GPU on your computer’s video adapter to speed screen redraw operations. For Photoshop to access the GPU, the video adapter must have a GPU that supports OpenGL. It also requires at least 128 MB of RAM, and a driver that supports OpenGL 2.0 and Shader Model 3.0.
So it sounds like the GPU won't be used for any processing, just for screen redrawing. (Is this anything more than getting a faster live preview?) In any case, any modern graphics card will meet these requirements. I'm leaning towards the HD 6770, which I think is comparable in performance to the GTX 550 Ti, but for some reason is much cheaper.