Guessing this one? http://www.argos.co.uk/static/Produc...LDING+BIKE.htm
I would concur with everyone else, don't spend money on any parts you can't easily move onto another bike if/when you can afford to upgrade.
Changing Pedals, saddle, tyres should be worthwhile, if handlebars are a big issue for you then change them. Folding handlebar stem would be a step too far. Cheap folding handlebars flex like hell anyway.