''The Ram is a Kingston 256 MB DDR. If I want to boost this do I have to match this or can I increase to what I want''
It is likely there will be a limit of 1Gb RAM - if you go to crucial memory and download the memory checker it will tell you what the Motherboard will support
''This computer hasnt had a DVD before, I installed one and it plays the sound but is asking for a decoder''
What are you using to play the DVD - try VLC player from
http://www.videolan.org/
It will play most things without worrying you for CODECS