i really suggest you do a search of this forum. you will find all the answers you need.
but to summarise, the p4 northwood now, particularly the 1.6A 1.8A and 2.0A. This will probably change when the thouroughbred comes out.
P.S. you didnt even say how you wanted to rate the O.C.
best by Max percentage or pure Mhz gain obtained?
Best performance per dollar spent?
Which is easiest to do?
Despite appearances im not Phsysic. I may need your system specifications to solve your problem!