Well if you want CUDA.. and are willing to spend 500$, wait a little while for the Nvidia GTX 300'S come out. (Fermi)
They will fit inside your budget, and will have superior features and CUDA support, or they will heavily lower current Nvidia lineup prices.
A little patience and you will get huge bang for your buck.
Of course if you have to make it now you could get a GTx 285 or even a 295within your budget. http://www.newegg.com/Product/Product.aspx?Item=N82E16814130480&cm_re=gtx_285-_-14-130-480-_-Product
< GTX 285 http://www.newegg.com/Product/Product.aspx?Item=N82E16814143167&cm_re=gtx_295-_-14-143-167-_-Product
< GTX 295
These are the most powerful cards for CUDA and if your willing to spend the money on them, they will last the longest.