Pc with XP home will be mainly used for internet, mail,a bit of dvd-cd burning,will adding another 1GB ddr2 ram be overkill for my use and could i save a few quid by not doing so.if i decide to get vista at a much much later date add more ram then,also if i buy just 1GB should i get 2x512mb or 1 stick of 1GB.
It will make no difference whatsoever for what you need. Get a 1Gb stick or 2x512...whichever is cheaper. Instead of another 1 Gb stick buy an external HD or a 2Gb memory stick for backups. These will be of much more use.
With your present system in use bring up the Windows Task Manager>Performance tab and you'll be able to see just how much memory is being used at any given moment. It will provide a guide as to what you may need.
I have 512MB and it's more than adequate for my purposes including use of a PCI TV card, although I don't have any interest in games(!)