Heyas I'm new around here, but I couldn't help but register and post here.
For £500 it is possible, but it also depends on what other applications or use you have for your PC.
EDIT
Unless I missed a important component out, you have about £50 to use to configure your build.
I'm pretty sure that photoshop is more CPU and RAM intensive then GPU intensive. Unless you absolutely need to upgrade, I recommend you wait and gather more funds to about £600-£700 as it will open up alot more choices, especially if you are planning to work long term with graphics. I.E preferably get a intel quad core, at least 4GB of RAM and a Nvidia Quadro FX/ATI Fire GL.