Any of those cards would offer an improvement over the 295 for those things that get accelerated. That doesn't necessarily mean your videos will benefit, as we have no way of knowing how much of your sequence is accelerated by the GPU.
Thanks. My impression is that nothing through the 500 series provides a substantial improvement over the 295 - the numbers seem to indicate the improvements would be moderate. Am I misreading things? Would the cost of a 500/600 series be worth the improvements over a 470/480?
I do have the list of accelerated operations and I use a good many of them.
Your CPU is likely the biggest bottleneck in your current rig. You can confirm this by monitoring your CPU usage (Task Manager / performance tab) and GPU usage (I use MSI Afterburner which may also work to monitor GPU on non-MSI graphics cards).
My vote would be save your cash and put it towards a new motherboard, cpu, and RAM when you have enough. Good Intel i7 options would range from around $600 to $1000 and would completely blow away your Athlon so far as performance for Premiere Pro CS5/5.5/6 goes.