Will this not run x16x16
http://www.overclockers.co.uk/showproduct.php?prodid=MB-476-AS&groupid=701&catid=5&subcat=1990
??
I am right in thinking my current board is 16x16x ?
Yes it is.
In CPU bound games you'll gain far more from moving to the i5-2500K than the 2% you lose by dropping to x8/x8.
But I wouldn't have though your current setup was exactly struggling.