Hi Dean,
Basically, the difference is that the more expensive cards have faster memory and processors with more cores and it will make a difference. However, you can use the one you have in your bottom link too.
So, if you are likely to upgrade to CS5 or upgrade your computer later, where you take your new graphics card with you to the next then the faster once would be one to pick, but if you are not likely to make any other changes for some time, just choose the cheaper option.
I wasn't trying to say to you to choose the most expensive one, but only to illustrate the spectra of choice, as I do not know your workflow if you work on small or big files, do pano's etc...
I myself picked in the middle of the range, actually at the lower middle range with my own graphics card the ASUS GTX-460, mainly because I do, do a lot of pano work where I can take advantage of the better graphics card with looking at the previews, my prefered choice would have been a card with 2 or more gigabytes of video ram.
the bottomline here is to find know what is going to make the biggest difference in boosting improved speed and quality in your work within a given set of restrictions vs $$$
Henrik