OK, well, my first impression is that this sentence in Japanese doesn't need to explicitly say the "I", as it would be understood by context, but for argument's sake, here's my take on it:
watashi ha, anata ga sugoi koto/no wo/ha shitteiru.
EDIT: My mental image of this sentence is someone being sarcastic. Also that something else is coming up, as in, "I know you're cool and all, but..."