TBH i follow a few simple rules wen it comes to vista,
dont use single core CPU's with vista,
dont use less than 1Gb RAM with vista,
and if you follow them you should be ok and as you can see your computer doesnt meet "my reccomended spec" so i wouldnt upgrade just yet.
DISCLAIMER: Above statements are not facts they are opinions & reccomendations and if you dont like em 'eat my vista'