"I have an Asus P5K/EPU motherboard will it run Crossfire mode as dual 16x mode or 16x/4x mode? anyone know?"
Yes one slot will run in 16X mode and one will run in 4X mode.
You can always check on ASUS's web page for the exact specs and download the manual (I always do this for the PC's I build and I put together a snapshot of the drivers and some small programs to throw on a CD.)
Anyway I hope this helps some
Edit: You can also check out the specs on this page, http://www.newegg.com/Product/Product.aspx?Item=N82E16813131277
Just click the specifications tab on the right hand side.