Installation question

I have a memory question...I have 2x 256 sticks in my system, I have three memory slots. I just bought a stick of pny from staples (another 256) and installed it in the third slot. I booted my machine, and I'm still only displaying 512mb ram. Do I have to turn on the third ram slot in bios? I look around and didn't see anything. My mobo is listed below, I'm downloading the manual as I type. Or is this stick of memory faulty?

some information on the memory would help but it's probly cause most mobos only support 2 sticks of ddr333+

hehe, sorry, its all pc2100, the initial two sticks are unknow brand running samsung chips, the other is pny, didn't pay attention to the chips.