I think that apart from a couple of programs that will use a reasonably decent graphics card anything that you upgrade to will be fine. What about the rest of your components? CPU and RAM for instance?
I'm using a cheap Geoforce GT 640 to run Photoshop CS6 and Lightroom and find it perfect. Mind you I do have an I5 quad core + 16Gb ram. This handles everything I throw at it, including up to 14 120Gb Tiff files for photo stacking
as above for editing Photos, its not so much a better card. But you do need a Fast CPU and plenty of Memory if dealing with large file. But why do you need Large File. My bet is you do not Print anything bigger than A4 in which case you need to reduce the picture size or Quality in which you capture Photos. From about 5meg Pixel it should be good enough for most peoples needs
Interesting points Woodchip, A faster chip in a month or two is my next step after the graphics card. Currently looking at an i5 and a minimum of 8gb of RAM. The choice of card was based on hssutton's using a similar card.
On photo sizes, I shot RAW, which is why the file sizes are large and often print A3.
I am always willing to heed advice and change things if needed.