Re: Athlon 64's and Memory Req's
Chances are, by the time you decide you wanted to upgrade your processor, the memory standards, motherboard standards and the like that would go hand in hand with a processor upgrade would have moved on. In which case your memory might well not be that useful. Having said that you can often use lesser spec memory in a system, it's just that you won't get the most out of it, and any slower memory sticks, if used with faster ones, if they work at all would slow all memory to the lowest common setting.
Biggest thing at the moment is whether or not, as purenuman says, your motherboard has dual memory slots. If it does, to maintain performance you need a pair of 512MB RAM sticks rather than a single 1GB stick.