What operating system do you run? And what kind of RAM do you have in the system now total? If you right click on my Computer and hit properties, what does it say under Computer for "X GB of RAM"?

PCs can be very finicky about memory, for example DDR2 RAM wants to be matched up 2 at a time, and putting in slower new RAM can cause your old RAM to drop down in timings. Theres a whole bunch of number settings like CAS/RAS that effect the timings, I went from 4-4-4-7 to 5-5-5-something when I moved from 2GB to 4GB because my newer sticks were slower than my older ones. But I run XP so it only sees 3GB of memory anyways