Ive built several Asus P4S333/P4S533 +1.6a combos..all have done 2133mhz with only a bump of CPU voltage to 1.55v, stock cooler and heatsink also..
Alot of 1.6a's can do 150fsb~2399mhz with some additional voltage (1.65v) and a decent cooler, even stock cooler can do 2399 but the temps are a bit high~56'c.