You machine come with 2GB of ram as standard, either in 2x1GB or 1x2GB.
As the MS-7525(Boston) motherboard supports 4GB and only has 2 DIMM sockets, it would be best to upgrade to 2x2GB PC2-6400 DIMMS, but remember that the version vista that came with the machine is 32-bit and will not see all of the memory, so upgrade to Windows 7 Home x64.
Plus if you have the case, look for a second hand Core 2 Duo E7xxx (Wolfdale-M) or Core 2 Duo E4x00 (Conroe) that will give a significant spee dboost over your existing chip which is normally a Pentium E2xxx series (Conroe).