More computer problems

Hey, one of my ram sticks was dead so i needed new ram.
I bought 2 gigs of ddr2 pc5300 and tried putting it in the 2nd slot (i have 4 slots) and it did not boot. I tried putting it in the third slot and it booted. I then tried reversing the 2 sticks and it did not boot. Why does this happen?
I bought 2 gigs of ddr2 pc5300 and tried putting it in the 2nd slot (i have 4 slots) and it did not boot. I tried putting it in the third slot and it booted. I then tried reversing the 2 sticks and it did not boot. Why does this happen?