The Dell specs for the Dell Dimension 3100 say that it takes a maximum of 2GB DDR2 RAM Memory, Unbuffered, Non-ECC.  However, you can install 4GB (that is 2 x 2GB DIMMS) and, with a 64-bit Windows Operating System installed, and your Computer Properties under Windows will show Installed Memory (RAM) 4.00 GB (2.99 GB usable).  4  GB (2 x 2GB DIMMs) PC2-5300, DDR2, 667Mhz, Unbuffered, Non-ECC RAM.