There will be a noticeable jump in performance, especially gaming and even some in web performance with a jump like that.
Just make sure that what you are buying is compatible with your mother board (pins and speed). You'll need to pull out the user manual or go online to see a list of what speeds will work.
Also, a lot of mobos won't take mixed speed ram. It will pick the higher of the two and just run that one, so don't bother leaving the 500 stick in if it isn't the same speed (i.e. pc3300) as what you are picking up.