This system uses a long obsolete type of RAM that wasn't on the market all that long -- RDRAM or Rambus DRAM.  Beyond its astronomical cost at the time, it hasn't been made in over a decade -- more like 15 years.  The only modules you're going to find will be used on EBay or similar, and there are many pitfalls in selecting RDRAM modules. Bottom line:  take whatever you'd spend on RDRAM and buy a newer system.  This one isn't worth the effort to upgrade -- it won't run Windows 10 in anything resembling reasonable fashion even if you max it out to 2G RAM (though that's a minimum for Windows 10, a more practical minimum, particularly with the slow CPU on this system is 8G -- which you can't upgrade to).  
Hi Everyone, this my first post here. I have an old Dell Dimension 8200 which is in very good shape. Evertything is doing well however it has only 512 MB Ram and it is running on Windows XP. I want to upgrade to Windows10 and  therefore I need an additional 512MB Ram (since the requirement for windows10 is at least 1 GB Ram). Anyone know how and where to find this memory? Thank in Advance LatinB  
Dimension 8200