Heh, I knew someone would ask about that eventually.
My friend was building his little brother a computer around the same time I was doing all those upgrades to mine (which was when I took these pics).
He insisted that his little brother's old computer had DDR3 in it, so he didn't order any to save money. But, of course, when the parts got there and he got everything together and pulled the RAM out of the old computer, he couldn't get it to fit.
He called me, I went over, and realized that it was DDR2, so I went back to my apartment, pulled 4GB (the missing sticks) outta mine, and let his little brother borrow them until he got RAM for himself.
All 6 slots are filled now. =P