sometimes when companies advertise products they over-exagerate how much is available. they did it with my ipod at least... and with the new external hdd i bought.
if thats not it, it could be a memory limit on your motherboard or something.
intel core-i7 920 4+ GHz with the thermaltech 120 extreme with adition 120mm fan lapped. mobo: asus p6t deluxe gpu: Sapphire 5870 OCED. PSU: power pc and cooling 910 watt RAM: Corsair 6GB ddr3 1600 HDD: two 640 Gb WB Caviar in RAID 0: coolermaster haf 932 with all red fans
monitor: samsung ToC T260 roseblack 25.5" 5ms 1920x1200