with older system you want to use a match pair of ram in the ram banks. on that memory advisior if you click on all memory it show you all ram that can go into the unit. what you need is go back and read the max memory the mb can run with. if memory advisor was right...8g per intel web page info on your p35 mb. so the max ram would be two 4g ram dims or 4 2g ram dimms.
if you install over 4g of ram you need a 64bit os to use that amount of ram. 32 bit os stop at 3.2g. for the money on an older pc running ddr-2 4 gig will be fine. anything over 4g is over kill on a older system.