You must have a lot of things going on. I'm using a GTX 650 with 2 GB of VRAM. Comparatively speaking, its performance is on par with the 550Ti (if I remember correctly).
Mine doesn't even hiccup while using the Mixer Brush. (At least in my testing; I primarily use Ambient Design's ArtRage 4.5 when I'm in the mood for painting, which taxes my system more than Photoshop does.)
There's always a debate at which graphics API is more efficient. Generally speaking, it's thought that OpenGL is a little better performance-wise.
Today's bleeding-edge card becomes tomorrow's obsolescence. Unless you have the cash to burn, I would go with a model a notch or two below it. The differences in speed would be so negligible in this case it wouldn't matter. IMHO, anyway.