Yeah if you on a budget, this would be a good deal.
However, it might just cheaper for you to get another stick of 512 for $80 instead of selling your old stick. I doubt anyone is going to buy that stick for more than $70 since you can get Patriot PC3200 2-3-2-5 for $80 on Newegg. So then you wont save much $ by selling and getting the value ram.