Biggest thing with memory is that you have the correct type.... such as ECC or NON-ECC, ( Most home computers use NON-ECC.
Then you have the correct size stick for the system, as for I have seen people accidentally purchase Laptop Memory for Desktops and the other way around.
Next FSB rating such as 266, 333, 400, 533, 667, 800Mhz, and so on and that they all match. * ( Sometimes you can underclock a stick to work with others such as adding a 400Mhz stick in a motherboard only rated for 333Mhz and paired with a 333Mhz stick, but its best to avoid mismatches to avoid problems. )
Next is checking to make sure that their latency is the same which is like CL2.5 or CL3 etc.
Next making sure the sticks do not exceed the maximum memory capacity per memory slot as specified in the motherboard or computer manufacturer support site. Such as some systems claim to support 4GB of RAM, but only 2GB per slot maximum, so you cant stuff a single 4GB stick into the motherboard, you need to instead get 2 x 2GB sticks for 4GB.
Lastly, if you have a system that is functional that can get online, you can run the crucial memory utility that will specify the memory your system should have in it. * Only warning here is that if it says you have 4 memory slots, be sure to check the motherboard to make sure it in fact does have 4 memory slots as for on a Biostar MCP6PB M2+ mATX motherboard that I have, it states that it can handle 4 sticks of memory, but there are only 2 memory slots available, so internally the memory controller may be reporting that it can address 4 slots, BUT only 2 slots are really available.
Here is the link to that Crucial memory Scan Utility:
http://www.crucial.com/usa/en/systemscanner*** If you want to mix memory and test to make sure that they are really running ok, you can download memtest86 and burn that to a CD etc and boot the system off of it and run it through about 3 to 5 full tests. If it starts to complain about memory addresses then you have a bad stick in the mix or they are not happy working together.
http://www.memtest86.com/download.htmAs far as performance advantages go... your not going to squeeze much out of this system for noticeable gain. But if your system has 266Mhz memory in it and the motherboard supports 333, then installing 333Mhz in place of the 266 does have a performance impact. Also, your motherboard may or may not have dual-channel support. If it does have dual-channel support then if you install matched size pairs of sticks the data bus between the CPU and Memory takes advantage of dual-channels and so it can process data between CPU and RAM faster with less of a bottleneck. Dual-Channel slots are usually marked by different color plastic indicating the slots that are paired. So if you have 2 black and 2 blue plastic memory slots and have 2 x 1GB sticks and 2 x 512MB sticks, you would stuff the 2 x 1GB sticks into say the blue slots and then the 2 x 512MB sticks into the black memory slots. Your system will then boot with 3GB of RAM that is Dual-Channel. *If all slots are the same color, its more than likely just regular single-channel unless otherwise specified in the motherboard or computer manufacturer support site. ** Dual-Channel requires that the sticks paired in the dual-channel specific slots are the same capacity. Also if you had 3 sticks and a 4 slot motherboard its best not to have say 2 x 1GB sticks paired in the blue slot and then a single 1GB stick in the black slot with 1 slot empty as for this kind of undoes the dual-channel and your really sort of running 3GB single-channel. I am using the 3GB size as a reference here because with many older systems that do not support 64-bit OS, people usually try to go for maximum memory which is 3GB for Windows XP 32-bit and Windows 7 32-bit for 32-bit CPU's.
