That Asus P4C800 board hits 280FSB WITH STOCK HEATSINK so spare your money for a NB heatsink please and keep the stock one for a measly 240FSB, so I am very suprised you cannot achieve this. ARe you sure other factors are not at issue like CPU voltage and power supply quality => stability?
If you are dead set on getting a NB cooler, any of the First 6 from left to right on the top Row will do the job.
Here
Zalman 7000AlCu is cheaper by about $20+ after having to buy a Thermalright anything.....and offers better cooling when running at <=25db in noise level. If you want Thermalright you pay extra for the best performance possible which of course comes at a cost of higher noise level. Even if Thermalright delievered slightly better temperatures by 1-2*C it costs $20 more which really does not justify its price at all.
Also, you willl have to clean both after about 3-4 months of 24/7 operation as the dust builds up and for 7000 you can simply scoop the dust with a Q-tip when you turn it off. For any thermalright product, you will have to either take it off every time you clean it or buy compressed air cans. Of course you could buy some air filters but still all these factors should be considered.
The reality is both of these will run the cpu and maintain stability so why pay $20 extra for a product that might perform better but lower temperature wont make your cpu last longer if you are not exceeding the threshold of thermal throttlign so...
Cheers.