http://www.memoryx.net/ibmtha3081me1.html

using the provided model number you have provided the memory you use for your computer is DDR1 not DDR2 but DDR1. so your computer has two dimms (stick of memory) which in this case you have a 256MB of DIMM (stick of memory) and a 512MB DIMM. The options is you can upgrade to 1GB  of memory by taking out the 256MB and putting in 512MB in it's place or if you want to upgrade to 1.5 you can put in 1GB in it's place. If you really wanna upgrade you can put in 1GB in both dimm slots and have a total of 2GB but if your using XP 1GB should be enough in most cases.

Some differences between a stick of DDR RAM and DDR2 RAM are they operate on different voltages with a different amount of current, DDR2 tends to perform better on less power in general, and the physical shape of the stick is slightly different so they won't fit in each other's slot type.

Robert Paradis
ConnectPro Online Support Expert