Yes, some memory is faster or more tolerant to harsh environmental factors. However, there is not one brand that is far superior to any other. Think about memory as you would ammo. Most will spend more to keep the older XP machine that the cost of replacement with something that is supported. Today, you should consider 6g memory and Windows 7/8 64 bit as the basic starting point. Office 2013 should hit rtm by end of the year.
With all of the exploits known in XP, you are not only putting your data at risk but also contributing to other issues that put all of us at risk. Ask yourself if you would leave a loaded gun sitting outside your front door?