Choose a graphic card for GPU CUDA?

Hi,
I am going to work on GPU programming, more specifically using CUDA (NVIDIA). Can you advise me on the perfect graphic card for CUDA. The budget is approximately 500$.
Thanks for your answers.

I have to develop some algorithms for computer vision. I have to learn, but it's for industrial applications so it's not just an "Hello World" program. I really have to implement complicate algorithms. These algorithms are slow in Matlab and C++ programming. So, we gonna try to speed up them.

It should be way faster if you look at nvidia site instead of asking for advices here http://www.nvidia.com/object/tesla [...] tions.html
Also look carefully at the additional info column, there're some important links there.

I've never programmed using a GPU (I just do regular CPU programming ) I did however do a parellel processing unit back at uni - one of the things is that if you do your program properly, it will scale.
Hence I would ASSUME (someone correct me if this is a wrong assumption) that a GT or a GTX would work the same, but the GTX would be quicker

I have read the CUDA specs and installed the package with the necessary drivers in the past. The ideal CUDA card is the GTX or the Ultra according to all I've read and an SLI setup of either is ultimate.

Find your CUDA literature and read it.