Bios "seeing" 2gb of RAM while SpeedFan sees 1.5gb

Hello all. My little bro has a MSI k8n neo4 motherboard with two 512mb sticks of GEIL RAM. He's had this for a couple of years. This year for his birthday, I bought him 1gb of GEIL RAM (tried to get roughly the same type).

No matter how he puts these sticks into the slots, only the 1gb and one of the 512mb is ever recognized in BIOS/WindowsXP at a time. He has booted up the PC with each of the individual sticks, so he knows they all work. He also has individual checked each of the 4 RAM slots.

Now the funny thing is that SpeedFan always correctly identifies the two 512mbs and the 1gb in the correct slots. Does anyone have any idea where the problem could be or direct me to a good forum where someone could help me out? Thanks!