Still not got that card?
The reason that PSU has 2x8 pin is it is so powerful, some cards need the extra pins because of their high power draw.
I take it the PSU is this?http://www.newegg.com/Product/Product.aspx?Item=N82E16817139005
If so; Good choice, sir! It will laugh at the HD4890, all you need to do is not to use the extra 2-pin addons (it has 2x6+2 connectors), just tape them out of the way.