I think they run on an honesty policy, that you are a system builder buying this, but you don't actually have to prove it. I don't think you have to buy anything with it, but you can always get your GPU at the same place if you want.
I don't think your CPU will be bottlenecking at all so you could get a GX2 if you want (lol).
This is what I found on your PSU:
+12V1 - 18A
+12V2 - 18A
+12A3 - 18A
Now what I don't know is if you add them together or only count one. This is the part that confuses me about PSUs. I'm not sure but I think the 9600GT needs 26A.