Memory upgrades, flash media, and usb storage at Crucial.com
the above link will tell you how much memory you can safely install into your system without overdoing it - they suggest the size you need etc just choose the "Crucial System Scanner tool" i have had no probs with my computer since
maybe Neil or Entwisi or Mick could take a peek and tell you whether it gives good info etc
not saying buy from there but at least you will have an idea what to buy and how much -good luck