i would go for the GTX680.
It will outperform the k5000 and cost you less money
both have CUDA but the 680 performance may be ~30% faster.
you can see some benchmarks here
you can buy a mac version of the card
if you buy the PC version you wont have a boot screen if you use it to drive your monitors (may or may not be that big of a deal to you)
680 is supported on windows, i dont recall if its in the 11.02 update on mac but if not then you can add to the supported cards txt file.
not sure why its a 6 minute video as it should take you about a minute to add to yout txt file