Similarly, you can go to a Web-site of any major RAM manufacturer (say Crucial or Corsair), choose your computer model and you will be offered a choice of compatible RAM modules.
I should mention I assumed that by Windows 7, Microsoft would have solved this issue, but after a blue screen of death, I realized that is was not solved. On all versions of Windows, under advanced system settings, performance, you need to adjust virtual memory. Note that I have Win 7 Pro and 4 GB of memory, but by default, this is set to automatic, and that Windows will manage your virtual memory. However, Windows is not AI, and is in fact not only stupid, it does not manage anything. It actually says that Windows recommends more virtual memory than what is allocated, but of course it does not change it even though its set to automatic. So, everyone, set this to manual. A good rule of thumb, set init size to 1.5 x your actual memory so for me I set it to 6144, and for maximum, set it to 2 times total RAM, so for me I set it to 8142, and no more blue screens of death since making this change, plus the computer now runs faster.
Update: Received and installed the new RAM The difference is incredible. Dell sure screwed up selling that machine with only 512K, or at least I screwed up buying it. Highly recommend more RAM for anyone running Vista. Now sitting with 512K module. I have an older machine running XP with only 1Gig of RAM. Going to add the 512 and see if it makes any difference.