| As LikeWhoa said, you should never mix 2 brands of memory, as well as 2 different types of memory from one brand. Even 2 memory sticks that are both labeled CL2 can still have differences in timing (like one having 2-2-2-3-1 Timing and the other having 2-2-2-4-1 Timing), and so on. Different sizes hurt, memory allocation can be problematic.
I guess you have 512MB Corsair, 512MB Kingston, 256MB Kingston... the worst case possible. I hope you have WindowsXP, as Win2k is very picky with memory. Running Win9X on a PC like yours would be idiotic, especially because it has a memory allocation error; anything above 256MB of Ram can and will cause all different types of bluescreens, sudden shutdowns and so on.
Do yourself a favor and sell the 256MB Kingston as well as one of the 512MB sticks that has the slower timing, and try to get another stick of the faster memory. If all have CL3 (CAS-Latency), sell all of 'em and get 2x512MB sticks with CL2. Faster CL timings are even more effective than a jump from DDR333 to DDR366.
It's most effective if the multiplyer of the CPU is the same as all latency-numbers together (2-2-2-4-1 would fit perfect for a multiplyer of 11). But that's only if you really wanna have a perfect system ^^ |