Can you give me buying advice of a 6970 card in a 27" iMac for just Photoshop, Lightroom, no video or games?
Well, it appears that some features of Photoshop CS5 may perform better with a good graphics card. One of these features that is particularly obvious is the new 3D functionality added to Photoshop. Also, there's an Adobe TV episode titled "Photoshop's Graphics Card Power Helps You Save Time". Here's a description of the episode:


  Use the power of your graphics card to navigate to any area of an image with new, ultra-smooth zooming and panning. Edit at the highest magnification and use the new Rotate View tool to smoothly turn your canvas for distortion-free viewing at any angle.


If you check out this Adobe Forums thread, they talk about the features that use the GPU (which can benefit from a better graphics card). Here are some of these features:


Mercury Graphics Engine: "The Mercury Graphics Engine (MGE) represents features that use video card, or GPU, acceleration. In Photoshop CS6, this new engine delivers near-instant results when editing with key tools such as Liquify, Warp, Lighting Effects and the Oil Paint filter. The new MGE delivers unprecedented responsiveness for a fluid feel as you work."
Adapative Wide Angle Filter
Liquify
Oil Paint
Warp and Puppet Warp
Field Blur, Iris Blur, and Tilt/Shift
Lighting Effects Gallery


Also, Adobe posted a blog entry titled "How to tune Photoshop CS5 for Peak Performance", and in it they say this:


  Photoshop is taking advantage of the powerful processor contained on these cards (called the graphics processing unit or GPU) to help speed up performance and activate a set of special GPU-accelerated features such as canvas rotation, panning and zooming, Adobe Repoussé, and Adobe Pixel Bender® filters.


So to sum it up, a better graphics card would greatly improve performance when doing 3D work with Repoussé and also with 2D work to an extent, but I guess you would have to determine for yourself if you're going to use Repoussé or the various filters that benefit from a better graphics card. In my opinion, if you can spare the money, then I would go for it, because you never know what you'll do in the future, but it's definitely not necessary to buy one.