I currently have an Xbox Live Gold membership that will expire in March 2011. However the price of membership is increasing to $59.99 starting November 1, 2010.
If I buy a 12 month membership now at the current price of $49.99, will the additional 12 months be added on to my existing membership or will I start a new membership which will expire 12 months after I buy it?
Yes, additional purchased membership time will stack. In addition to the one-year lock-in price currently offered by Microsoft, you can buy scratch cards at pretty much any retailer where games are sold. These can be bought at the current subscription price, and redeemed either now (and stacked with existing time) or sold to your friends later for a teeny tiny profit.
EDIT: Yes, it is stacked to my knowledge.
You can view this in the "Account section" of your home screen dashboard under "Settings", "Account".
Also, if you want to remove the "Auto renewal" and remove the Credit card from the account you will have to call Microsoft and they may take 3-7 days. There may be an option somewhere in the accounts.microsoft section of your live account although if the card is associated with the membership, it will not be able to be removed without the gold membership being removed...