your RAM may make some problems as 875 chip is little moody. good thing you have matched DDR's
secondaly, are you gonan install this board in your HP?
thirdly, this board will support your Northwood core CPU even at 400Mhz FSB. as far as your processor is with 478pins.
those telling that it will not support your processor are relying on Retailors quotes, not manufacturers. as 865 is somewhat similar chip and is supporting 400, you can as well have peek on Asus web siet being manufacturer that this board with 875p chipset supports 400Mhz FSB processors:
as wel, you can see on 875p chipset information on intel's site:
where is is clearly written, quoting the words: "Supports the highest performance Intel® Pentium® 4 processors plus, the flexibility to support other 478-pin Intel® processors.".
intel havent mentioned in their paper about 400Mhz systems buses as this boards with 875 chipset are intended to be installed with more powerful processors on P-4 core as this board qualifies for entry level work stations.
therefore, this chip definately will work with 400FSB processor (478-pin packages). I can say it fully assutred.
if you do not feel like comfortable, then you can get board on 865 series chipset. as both 865 and 875 were flag-ship chipsets in intel's history.