Any of the lower-end Radeon HD 5xxx series will work, I should think. Honestly, any somewhat-recent two-output video card will work. You only need the high-end stuff when you get to 3d rendering. Since neither movies, nor Photoshop have anything 3d, then any video card will work, as long as it can support the resolution you want to go to.
I can see things going wrong in two areas:
1. Not enough power. But its hard to tell if you need a new power supply or not if we don't know how much power you system is currently using.
2. Not backwards compatible with PCIe 1.0. Your board has a PCIe 1.0 slot, while video cards now are built on PCIe 2.0 specifications. Most PCIe 2.0 cards I've come across will also work on PCIe 1.0 slots, but you never know.