X51 R2, what memory do I need?

I've been experiencing blue screen unexpectedly for about a week now. On my 5 year old x51 r2. Previously had no issues despite its age. I initially thought it was a hard drive issue but having run diagnostic it shows the memory is the issue. What do I need to look for when buying new memory as in spec and size? I know how to install it but just want to make sure on getting the right thing before I buy.