cas,

They're using DELL'S OWN memory table, did you bother to read the page at all? Also, that table is based off of Vista's useage, not XP's. Windows XP wants about 320MB of RAM for the OS's personal use, Vista, wants slightly less than 700MB if you follow a basic install (i.e. not turning off alot of options in the "advanced" section where most users shouldn't tread).