PC Question, Please Help Me!

I need to buy more memory for my PC. I currently have 512mb of DDR Ram in 2×256 sims, but I don’t know the speed of it. I’ve had a look on the sims and there is some writing on the chips but I think these must be part numbers. I think the speed may be 133Mhz but how can you tell? I know the latest Ram runs at 400Mhz and have been told that you have to run all memory at the same speed.

Someone please help me!

Thanks, Plazz